diff --git a/src/app/api/mpesa/callback/route.ts b/src/app/api/mpesa/callback/route.ts
index 653bbc6..c7a84c0 100644
--- a/src/app/api/mpesa/callback/route.ts
+++ b/src/app/api/mpesa/callback/route.ts
@@ -2,6 +2,7 @@ import { NextRequest, NextResponse } from "next/server";
 import { connectDb } from "@/lib/server/db";
 import { isMpesaEnabled } from "@/lib/server/env";
 import { Transaction } from "@/lib/server/models/Transaction";
+import { recordSaleMovement } from "@/lib/server/stock";
 import { MpesaCallbackLog } from "@/lib/server/models/MpesaCallbackLog";
 
 export const runtime = "nodejs";
@@ -99,6 +100,7 @@ export async function POST(req: NextRequest) {
   txn.mpesaDetails.resultDesc = resultDesc;
   txn.mpesaDetails.receiptNumber = mpesaReceiptNumber;
   await txn.save();
+  await recordSaleMovement(txn);
 
   await MpesaCallbackLog.create({
     sourceIp,
diff --git a/src/app/api/mpesa/status/[transactionId]/route.ts b/src/app/api/mpesa/status/[transactionId]/route.ts
index 3e2d8d9..a04ec25 100644
--- a/src/app/api/mpesa/status/[transactionId]/route.ts
+++ b/src/app/api/mpesa/status/[transactionId]/route.ts
@@ -1,9 +1,11 @@
 import { NextRequest, NextResponse } from "next/server";
+import { isValidObjectId } from "mongoose";
 import { connectDb } from "@/lib/server/db";
 import { isMpesaEnabled } from "@/lib/server/env";
 import { Transaction } from "@/lib/server/models/Transaction";
 import { getAuth } from "@/lib/server/auth";
 import { queryStkStatus } from "@/lib/server/mpesa";
+import { recordSaleMovement } from "@/lib/server/stock";
 
 export const runtime = "nodejs";
 export const dynamic = "force-dynamic";
@@ -14,6 +16,7 @@ export async function GET(req: NextRequest, { params }: { params: { transactionI
   const auth = getAuth(req);
   if (!auth) return NextResponse.json({ error: "Not authenticated" }, { status: 401 });
 
+  if (!isValidObjectId(params.transactionId)) return NextResponse.json({ error: "Transaction not found" }, { status: 404 });
   const txn = await Transaction.findOne({ _id: params.transactionId, businessId: auth.businessId });
   if (!txn) return NextResponse.json({ error: "Transaction not found" }, { status: 404 });
 
@@ -29,6 +32,7 @@ export async function GET(req: NextRequest, { params }: { params: { transactionI
           fresh.mpesaDetails.resultCode = code;
           fresh.mpesaDetails.resultDesc = result.ResultDesc;
           await fresh.save();
+          await recordSaleMovement(fresh);
           return NextResponse.json(fresh);
         }
       }
diff --git a/src/app/api/purchases/route.ts b/src/app/api/purchases/route.ts
index 00f6fc1..6bd08c5 100644
--- a/src/app/api/purchases/route.ts
+++ b/src/app/api/purchases/route.ts
@@ -5,6 +5,7 @@ import { Purchase } from "@/lib/server/models/Purchase";
 import { Product } from "@/lib/server/models/Product";
 import { Branch } from "@/lib/server/models/Branch";
 import { objectId } from "@/lib/server/validation";
+import { recordMovement } from "@/lib/server/stock";
 import { authorize, branchParam, MANAGERS } from "@/lib/server/auth";
 
 export const runtime = "nodejs";
@@ -57,8 +58,9 @@ const createSchema = z.object({
 
 // POST /api/purchases — records a supplier purchase. The buying price is
 // snapshotted (unitCost/totalCost) so historical records never shift when
-// product prices change later. A completed purchase conceptually increases
-// branch stock.
+// product prices change later. A purchase of a catalogue product adds its
+// quantity to the branch's stock ledger; a free-text item (no productId)
+// is recorded as a cost only.
 export async function POST(req: NextRequest) {
   await connectDb();
   const gate = authorize(req, MANAGERS);
@@ -125,5 +127,18 @@ export async function POST(req: NextRequest) {
     recordedBy: auth.sub,
   });
 
+  if (productId) {
+    await recordMovement({
+      businessId: auth.businessId,
+      branchId: branchId!,
+      productId,
+      type: "PURCHASE",
+      quantity: parsed.data.quantity,
+      refId: purchase._id.toString(),
+      note: `From ${parsed.data.supplierName}`,
+      recordedBy: auth.sub,
+    });
+  }
+
   return NextResponse.json({ purchase }, { status: 201 });
 }
diff --git a/src/app/api/stock/adjustments/route.ts b/src/app/api/stock/adjustments/route.ts
new file mode 100644
index 0000000..06f5fd8
--- /dev/null
+++ b/src/app/api/stock/adjustments/route.ts
@@ -0,0 +1,83 @@
+import { NextRequest, NextResponse } from "next/server";
+import { z } from "zod";
+import { connectDb } from "@/lib/server/db";
+import { Branch } from "@/lib/server/models/Branch";
+import { Product } from "@/lib/server/models/Product";
+import { StockMovement } from "@/lib/server/models/StockMovement";
+import { authorize, MANAGERS } from "@/lib/server/auth";
+import { objectId } from "@/lib/server/validation";
+import { clientRefId, recordMovement, stockOf } from "@/lib/server/stock";
+
+export const runtime = "nodejs";
+
+const adjustmentSchema = z.object({
+  clientUuid: z.string().uuid(),
+  branchId: objectId.optional(), // owners must give one; managers default to their own branch
+  productId: objectId,
+  // COUNT: "we physically counted N", so the ledger is corrected to N.
+  // DELTA: "add or remove N", e.g. opening stock, breakage, or a delivery
+  //        that was not recorded as a purchase.
+  mode: z.enum(["COUNT", "DELTA"]),
+  quantity: z.number().int().min(-1_000_000).max(1_000_000),
+  note: z.string().trim().max(300).optional(),
+});
+
+// POST /api/stock/adjustments — managers (own branch) and owners (any branch).
+export async function POST(req: NextRequest) {
+  await connectDb();
+  const gate = authorize(req, MANAGERS);
+  if (!gate.ok) return gate.response;
+  const auth = gate.auth;
+
+  const parsed = adjustmentSchema.safeParse(await req.json().catch(() => null));
+  if (!parsed.success) {
+    return NextResponse.json({ error: parsed.error.issues[0]?.message ?? "Invalid request" }, { status: 400 });
+  }
+  const data = parsed.data;
+  if (data.mode === "COUNT" && data.quantity < 0) {
+    return NextResponse.json({ error: "A counted quantity cannot be negative" }, { status: 400 });
+  }
+
+  let branchId: string | null;
+  if (auth.role === "SUPER_ADMIN") {
+    branchId = data.branchId ?? null;
+    if (!branchId) return NextResponse.json({ error: "branchId is required" }, { status: 400 });
+  } else {
+    branchId = auth.branchId;
+    if (data.branchId && data.branchId !== auth.branchId) {
+      return NextResponse.json({ error: "Cannot adjust stock for another branch" }, { status: 403 });
+    }
+  }
+  if (!branchId) return NextResponse.json({ error: "No branch assignment for this account" }, { status: 403 });
+
+  const [branch, product] = await Promise.all([
+    Branch.exists({ _id: branchId, businessId: auth.businessId }),
+    Product.exists({ _id: data.productId, businessId: auth.businessId }),
+  ]);
+  if (!branch) return NextResponse.json({ error: "Branch not found in your business" }, { status: 404 });
+  if (!product) return NextResponse.json({ error: "Product not found in your business" }, { status: 404 });
+
+  // A retried submit with the same clientUuid is a no-op.
+  const refId = clientRefId(auth.businessId, data.clientUuid);
+  const already = await StockMovement.exists({ type: "ADJUSTMENT", refId });
+  if (!already) {
+    const before = await stockOf(auth.businessId, branchId, data.productId);
+    const delta = data.mode === "COUNT" ? data.quantity - before : data.quantity;
+    if (delta !== 0) {
+      const label = data.mode === "COUNT" ? `Count: ${before} -> ${data.quantity}` : "Adjustment";
+      await recordMovement({
+        businessId: auth.businessId,
+        branchId,
+        productId: data.productId,
+        type: "ADJUSTMENT",
+        quantity: delta,
+        refId,
+        note: data.note ? `${label}. ${data.note}` : label,
+        recordedBy: auth.sub,
+      });
+    }
+  }
+
+  const quantity = await stockOf(auth.businessId, branchId, data.productId);
+  return NextResponse.json({ branchId, productId: data.productId, quantity }, { status: 201 });
+}
diff --git a/src/app/api/stock/movements/route.ts b/src/app/api/stock/movements/route.ts
new file mode 100644
index 0000000..5baa387
--- /dev/null
+++ b/src/app/api/stock/movements/route.ts
@@ -0,0 +1,31 @@
+import { NextRequest, NextResponse } from "next/server";
+import { connectDb } from "@/lib/server/db";
+import { StockMovement } from "@/lib/server/models/StockMovement";
+import { authorize, branchParam, MANAGERS } from "@/lib/server/auth";
+
+export const runtime = "nodejs";
+export const dynamic = "force-dynamic";
+
+// GET /api/stock/movements?branchId=&limit= — recent ledger entries, newest first.
+export async function GET(req: NextRequest) {
+  await connectDb();
+  const gate = authorize(req, MANAGERS);
+  if (!gate.ok) return gate.response;
+  const auth = gate.auth;
+
+  const scope = branchParam(auth, req.nextUrl.searchParams.get("branchId"));
+  if (!scope.ok) return scope.response;
+  const limit = Math.min(Number(req.nextUrl.searchParams.get("limit")) || 50, 200);
+
+  const filter: Record<string, unknown> = { businessId: auth.businessId };
+  if (scope.branchId) filter.branchId = scope.branchId;
+
+  const movements = await StockMovement.find(filter)
+    .sort({ createdAt: -1 })
+    .limit(limit)
+    .populate("branchId", "branchName")
+    .populate("productId", "label unit")
+    .populate("recordedBy", "name")
+    .lean();
+  return NextResponse.json({ movements });
+}
diff --git a/src/app/api/stock/route.ts b/src/app/api/stock/route.ts
new file mode 100644
index 0000000..190cab8
--- /dev/null
+++ b/src/app/api/stock/route.ts
@@ -0,0 +1,24 @@
+import { NextRequest, NextResponse } from "next/server";
+import { connectDb } from "@/lib/server/db";
+import { authorize, branchParam } from "@/lib/server/auth";
+import { getStockLevels } from "@/lib/server/stock";
+
+export const runtime = "nodejs";
+export const dynamic = "force-dynamic";
+
+// GET /api/stock?branchId= — current stock per branch and product.
+// Owners see every branch (or one, via branchId); everyone else sees only
+// their own branch.
+export async function GET(req: NextRequest) {
+  await connectDb();
+  const gate = authorize(req);
+  if (!gate.ok) return gate.response;
+  const auth = gate.auth;
+
+  const scope = branchParam(auth, req.nextUrl.searchParams.get("branchId"));
+  if (!scope.ok) return scope.response;
+  if (auth.role !== "SUPER_ADMIN" && !scope.branchId) return NextResponse.json({ levels: [] });
+
+  const levels = await getStockLevels(auth.businessId, scope.branchId);
+  return NextResponse.json({ levels });
+}
diff --git a/src/app/api/stock/transfers/route.ts b/src/app/api/stock/transfers/route.ts
new file mode 100644
index 0000000..b843ee6
--- /dev/null
+++ b/src/app/api/stock/transfers/route.ts
@@ -0,0 +1,136 @@
+import { NextRequest, NextResponse } from "next/server";
+import { z } from "zod";
+import { connectDb } from "@/lib/server/db";
+import { Branch } from "@/lib/server/models/Branch";
+import { Product } from "@/lib/server/models/Product";
+import { StockTransfer, IStockTransfer } from "@/lib/server/models/StockTransfer";
+import { authorize, branchParam, MANAGERS } from "@/lib/server/auth";
+import { objectId } from "@/lib/server/validation";
+import { recordMovement, stockOf } from "@/lib/server/stock";
+
+export const runtime = "nodejs";
+export const dynamic = "force-dynamic";
+
+// GET /api/stock/transfers?branchId= — recent transfers into or out of the branch(es) in scope.
+export async function GET(req: NextRequest) {
+  await connectDb();
+  const gate = authorize(req, MANAGERS);
+  if (!gate.ok) return gate.response;
+  const auth = gate.auth;
+
+  const scope = branchParam(auth, req.nextUrl.searchParams.get("branchId"));
+  if (!scope.ok) return scope.response;
+
+  const filter: Record<string, unknown> = { businessId: auth.businessId };
+  if (scope.branchId) filter.$or = [{ fromBranchId: scope.branchId }, { toBranchId: scope.branchId }];
+
+  const transfers = await StockTransfer.find(filter)
+    .sort({ createdAt: -1 })
+    .limit(100)
+    .populate("fromBranchId", "branchName")
+    .populate("toBranchId", "branchName")
+    .populate("productId", "label unit")
+    .populate("recordedBy", "name")
+    .lean();
+  return NextResponse.json({ transfers });
+}
+
+const transferSchema = z.object({
+  clientUuid: z.string().uuid(),
+  fromBranchId: objectId.optional(), // owners must give one; managers send from their own branch
+  toBranchId: objectId,
+  productId: objectId,
+  quantity: z.number().int().min(1).max(1_000_000),
+  note: z.string().trim().max(300).optional(),
+});
+
+/** Both ledger sides of a transfer. Idempotent, so it is safe to re-run on a retry. */
+async function applyTransfer(t: IStockTransfer) {
+  const common = {
+    businessId: t.businessId,
+    productId: t.productId,
+    refId: t._id.toString(),
+    recordedBy: t.recordedBy,
+    note: t.note,
+  };
+  await recordMovement({ ...common, branchId: t.fromBranchId, type: "TRANSFER_OUT", quantity: -t.quantity });
+  await recordMovement({ ...common, branchId: t.toBranchId, type: "TRANSFER_IN", quantity: t.quantity });
+}
+
+// POST /api/stock/transfers — move stock from one branch to another.
+// Owners: between any two branches. Managers: out of their own branch only.
+export async function POST(req: NextRequest) {
+  await connectDb();
+  const gate = authorize(req, MANAGERS);
+  if (!gate.ok) return gate.response;
+  const auth = gate.auth;
+
+  const parsed = transferSchema.safeParse(await req.json().catch(() => null));
+  if (!parsed.success) {
+    return NextResponse.json({ error: parsed.error.issues[0]?.message ?? "Invalid request" }, { status: 400 });
+  }
+  const data = parsed.data;
+
+  // A retried submit returns the transfer that already happened.
+  const existing = await StockTransfer.findOne({ businessId: auth.businessId, clientUuid: data.clientUuid });
+  if (existing) {
+    await applyTransfer(existing);
+    return NextResponse.json({ transfer: existing }, { status: 200 });
+  }
+
+  let fromBranchId: string | null;
+  if (auth.role === "SUPER_ADMIN") {
+    fromBranchId = data.fromBranchId ?? null;
+    if (!fromBranchId) return NextResponse.json({ error: "fromBranchId is required" }, { status: 400 });
+  } else {
+    fromBranchId = auth.branchId;
+    if (data.fromBranchId && data.fromBranchId !== auth.branchId) {
+      return NextResponse.json(
+        { error: "Branch managers can only transfer stock out of their own branch" },
+        { status: 403 }
+      );
+    }
+  }
+  if (!fromBranchId) return NextResponse.json({ error: "No branch assignment for this account" }, { status: 403 });
+  if (fromBranchId === data.toBranchId) {
+    return NextResponse.json({ error: "Choose two different branches" }, { status: 400 });
+  }
+
+  const [branchCount, product] = await Promise.all([
+    Branch.countDocuments({ _id: { $in: [fromBranchId, data.toBranchId] }, businessId: auth.businessId }),
+    Product.exists({ _id: data.productId, businessId: auth.businessId }),
+  ]);
+  if (branchCount !== 2) return NextResponse.json({ error: "Branch not found in your business" }, { status: 404 });
+  if (!product) return NextResponse.json({ error: "Product not found in your business" }, { status: 404 });
+
+  // Transfers are made online by a manager, so unlike offline sales they
+  // can be refused when the sending branch does not have the stock.
+  const available = await stockOf(auth.businessId, fromBranchId, data.productId);
+  if (available < data.quantity) {
+    return NextResponse.json(
+      { error: `Only ${Math.max(available, 0)} in stock at the sending branch`, available },
+      { status: 409 }
+    );
+  }
+
+  let transfer: IStockTransfer;
+  try {
+    transfer = await StockTransfer.create({
+      businessId: auth.businessId,
+      fromBranchId,
+      toBranchId: data.toBranchId,
+      productId: data.productId,
+      quantity: data.quantity,
+      clientUuid: data.clientUuid,
+      note: data.note ?? null,
+      recordedBy: auth.sub,
+    });
+  } catch (err: unknown) {
+    // Two identical submits raced and the other one created it.
+    if ((err as { code?: number })?.code !== 11000) throw err;
+    transfer = (await StockTransfer.findOne({ businessId: auth.businessId, clientUuid: data.clientUuid }))!;
+  }
+  await applyTransfer(transfer);
+
+  return NextResponse.json({ transfer }, { status: 201 });
+}
diff --git a/src/app/api/transactions/cash/route.ts b/src/app/api/transactions/cash/route.ts
index 67a41a4..b87db58 100644
--- a/src/app/api/transactions/cash/route.ts
+++ b/src/app/api/transactions/cash/route.ts
@@ -1,29 +1,67 @@
 import { NextRequest, NextResponse } from "next/server";
 import { z } from "zod";
 import { connectDb } from "@/lib/server/db";
-import { Transaction } from "@/lib/server/models/Transaction";
+import { Transaction, ITransaction } from "@/lib/server/models/Transaction";
 import { Product, BranchPricing } from "@/lib/server/models/Product";
-import { getAuth, requireRole, resolveBranchScope } from "@/lib/server/auth";
+import { Branch } from "@/lib/server/models/Branch";
+import { authorize } from "@/lib/server/auth";
 import { normalizeKenyanPhone } from "@/lib/server/phone";
+import { objectId } from "@/lib/server/validation";
+import { recordSaleMovement } from "@/lib/server/stock";
+import { MAX_LINES_PER_SALE, MAX_SALES_PER_REQUEST, type SaleResult } from "@/lib/syncProtocol";
 
 export const runtime = "nodejs";
 
-const saleSchema = z.object({
-  clientUuid: z.string().uuid(),
-  customerName: z.string().trim().max(120).optional(),
-  customerPhone: z.string().trim().min(9).max(15),
-  productId: z.string().min(1),
+const lineSchema = z.object({
+  productId: objectId,
   quantity: z.number().int().min(1).max(500),
-  clientSubmittedAt: z.string().datetime().optional(),
+  unitPrice: z.number().nonnegative().optional(), // what the device showed; informational
 });
-const bodySchema = z.object({ sales: z.array(saleSchema).min(1).max(100) });
 
+const saleSchema = z.preprocess(
+  // Older clients sent one product per sale at the top level.
+  (raw) => {
+    const r = raw as Record<string, unknown> | null;
+    if (r && !Array.isArray(r.lines) && r.productId) {
+      return { ...r, lines: [{ productId: r.productId, quantity: r.quantity }] };
+    }
+    return raw;
+  },
+  z.object({
+    clientUuid: z.string().uuid(),
+    staffId: objectId.optional(), // who recorded it on the device
+    branchId: objectId.optional(), // where it was recorded
+    customerName: z.string().trim().max(120).optional(),
+    customerPhone: z.string().trim().max(20).optional(),
+    clientSubmittedAt: z.string().datetime().optional(),
+    lines: z.array(lineSchema).min(1).max(MAX_LINES_PER_SALE),
+  })
+);
+type SaleInput = z.infer<typeof saleSchema>;
+
+const bodySchema = z.object({ sales: z.array(z.unknown()).min(1).max(MAX_SALES_PER_REQUEST) });
+
+/**
+ * POST /api/transactions/cash — record cash sales, one or many, live or
+ * replayed from the device's offline queue.
+ *
+ * Every sale gets its own result so the device knows exactly what to do
+ * with each queued row:
+ *   saved      newly recorded           -> mark synced
+ *   duplicate  already recorded earlier -> mark synced (this is a replay)
+ *   rejected   can never be recorded    -> stop retrying, show the cashier
+ * One bad sale never blocks the others in the batch.
+ *
+ * Idempotency: each line's clientUuid is `${saleUuid}:${index}` and is
+ * unique in the database; the stock movement for each line is keyed on
+ * the transaction id. Replaying a sale any number of times records it,
+ * and deducts its stock, exactly once.
+ */
 export async function POST(req: NextRequest) {
   await connectDb();
-  const auth = getAuth(req);
-  if (!requireRole(auth, "DELIVERY", "BRANCH_MANAGER")) {
-    return NextResponse.json({ error: "Not authenticated" }, { status: 401 });
-  }
+  const gate = authorize(req, ["DELIVERY", "BRANCH_MANAGER"]);
+  if (!gate.ok) return gate.response;
+  const auth = gate.auth;
 
   const json = await req.json().catch(() => null);
   const normalized = Array.isArray(json?.sales) ? json : { sales: [json] };
@@ -31,61 +69,124 @@ export async function POST(req: NextRequest) {
   if (!parsed.success) {
     return NextResponse.json({ error: parsed.error.issues[0]?.message ?? "Invalid request" }, { status: 400 });
   }
-
-  const branchId = resolveBranchScope(auth!);
-  if (!branchId) {
+  if (!auth.branchId) {
     return NextResponse.json({ error: "No active branch assignment for this account" }, { status: 403 });
   }
 
-  const { sales } = parsed.data;
-  const productIds = Array.from(new Set(sales.map((s) => s.productId)));
-  const products = await Product.find({ _id: { $in: productIds }, businessId: auth!.businessId }).lean();
-  const overrides = await BranchPricing.find({ branchId, productId: { $in: productIds }, businessId: auth!.businessId }).lean();
-  const productMap = new Map(products.map((p) => [p._id.toString(), p]));
-  const overrideMap = new Map(overrides.map((o) => [o.productId.toString(), o.unitPrice]));
-
-  const rejected: { clientUuid: string; error: string }[] = [];
-  const inserted: any[] = [];
-
-  for (const sale of sales) {
-    let phone: string;
-    try {
-      phone = normalizeKenyanPhone(sale.customerPhone);
-    } catch (e: any) {
-      rejected.push({ clientUuid: sale.clientUuid, error: e.message });
-      continue;
-    }
-    const product = productMap.get(sale.productId);
-    if (!product || !product.isActive) {
-      rejected.push({ clientUuid: sale.clientUuid, error: "Product not available" });
-      continue;
-    }
-    const unitPrice = overrideMap.get(sale.productId) ?? product.unitPrice;
-
-    try {
-      const created = await Transaction.create({
-        businessId: auth!.businessId,
-        branchId,
-        staffId: auth!.sub,
-        customerName: sale.customerName ?? null,
-        customerPhone: phone,
-        productId: product._id,
-        sizeLiters: product.sizeLiters,
-        quantity: sale.quantity,
-        unitPrice,
-        amountTotal: Math.round(sale.quantity * unitPrice * 100) / 100,
-        paymentMethod: "CASH",
-        paymentStatus: "SUCCESS",
-        clientUuid: sale.clientUuid,
-        syncedFromOffline: !!sale.clientSubmittedAt,
-        clientSubmittedAt: sale.clientSubmittedAt ? new Date(sale.clientSubmittedAt) : null,
-      });
-      inserted.push(created);
-    } catch (err: any) {
-      if (err?.code === 11000) continue;
-      rejected.push({ clientUuid: sale.clientUuid, error: "Could not save this sale" });
+  const sales: SaleInput[] = [];
+  const results: SaleResult[] = [];
+  for (const raw of parsed.data.sales) {
+    const sale = saleSchema.safeParse(raw);
+    if (sale.success) {
+      sales.push(sale.data);
+    } else {
+      const clientUuid = String((raw as { clientUuid?: unknown })?.clientUuid ?? "");
+      results.push({ clientUuid, status: "rejected", error: sale.error.issues[0]?.message ?? "Invalid sale" });
     }
   }
 
-  return NextResponse.json({ inserted, rejected });
+  // Tenant data needed to price every line in the batch, in three queries.
+  const productIds = Array.from(new Set(sales.flatMap((s) => s.lines.map((l) => l.productId))));
+  const branchIds = Array.from(new Set([auth.branchId, ...sales.map((s) => s.branchId ?? auth.branchId!)]));
+  const [products, overrides, branches] = await Promise.all([
+    Product.find({ _id: { $in: productIds }, businessId: auth.businessId }).lean(),
+    BranchPricing.find({ branchId: { $in: branchIds }, productId: { $in: productIds }, businessId: auth.businessId }).lean(),
+    Branch.find({ _id: { $in: branchIds }, businessId: auth.businessId }).select("_id").lean(),
+  ]);
+  const productMap = new Map(products.map((p) => [p._id.toString(), p]));
+  const overrideMap = new Map(overrides.map((o) => [`${o.branchId}:${o.productId}`, o.unitPrice]));
+  const tenantBranches = new Set(branches.map((b) => b._id.toString()));
+
+  for (const sale of sales) {
+    results.push(await recordSale(sale));
+  }
+
+  async function recordSale(sale: SaleInput): Promise<SaleResult> {
+    const reject = (error: string): SaleResult => ({ clientUuid: sale.clientUuid, status: "rejected", error });
+
+    // A queued sale belongs to the cashier who made it. The device only
+    // sends its signed-in user's rows; anything else is refused rather than
+    // credited to the wrong person.
+    if (sale.staffId && sale.staffId !== auth.sub) {
+      return reject("This sale was recorded by a different user. Sign in as them to sync it.");
+    }
+    // The sale stays with the branch where it happened, even if the
+    // cashier has since been moved — as long as that branch is in this business.
+    const branchId = sale.branchId ?? auth.branchId!;
+    if (!tenantBranches.has(branchId)) return reject("This sale's branch is not part of your business.");
+
+    for (const line of sale.lines) {
+      if (!productMap.has(line.productId)) return reject("A product in this sale no longer exists.");
+    }
+
+    // A bad phone number must not lose a sale that physically happened.
+    let customerPhone: string | null = null;
+    if (sale.customerPhone) {
+      try {
+        customerPhone = normalizeKenyanPhone(sale.customerPhone);
+      } catch {
+        customerPhone = null;
+      }
+    }
+
+    let newLines = 0;
+    let amountTotal = 0;
+    let priceMismatch = false;
+    for (const [index, line] of sale.lines.entries()) {
+      // A product deactivated while the device was offline is still
+      // recorded at its price: the sale already happened.
+      const product = productMap.get(line.productId)!;
+      const unitPrice = overrideMap.get(`${branchId}:${line.productId}`) ?? product.unitPrice;
+      const lineTotal = Math.round(line.quantity * unitPrice * 100) / 100;
+      const mismatch = line.unitPrice !== undefined && line.unitPrice !== unitPrice;
+
+      let txn: ITransaction;
+      try {
+        txn = await Transaction.create({
+          businessId: auth.businessId,
+          branchId,
+          staffId: auth.sub,
+          customerName: sale.customerName || null,
+          customerPhone,
+          productId: product._id,
+          sizeLiters: product.sizeLiters,
+          quantity: line.quantity,
+          unitPrice,
+          amountTotal: lineTotal,
+          paymentMethod: "CASH",
+          paymentStatus: "SUCCESS",
+          clientUuid: `${sale.clientUuid}:${index}`,
+          saleId: sale.clientUuid,
+          lineIndex: index,
+          syncedFromOffline: !!sale.clientSubmittedAt,
+          clientSubmittedAt: sale.clientSubmittedAt ? new Date(sale.clientSubmittedAt) : null,
+          clientUnitPrice: line.unitPrice ?? null,
+          priceMismatch: mismatch,
+        });
+        newLines += 1;
+      } catch (err: unknown) {
+        if ((err as { code?: number })?.code !== 11000) throw err;
+        // Already recorded by an earlier attempt: reuse it.
+        const existing = await Transaction.findOne({ clientUuid: `${sale.clientUuid}:${index}` });
+        if (!existing || existing.businessId.toString() !== auth.businessId) {
+          return reject("This sale id is already in use.");
+        }
+        txn = existing;
+      }
+      // Idempotent, and also repairs a line whose stock movement was not
+      // written because an earlier attempt failed midway.
+      await recordSaleMovement(txn);
+      amountTotal += txn.amountTotal;
+      priceMismatch = priceMismatch || txn.priceMismatch;
+    }
+
+    return {
+      clientUuid: sale.clientUuid,
+      status: newLines === 0 ? "duplicate" : "saved",
+      amountTotal: Math.round(amountTotal * 100) / 100,
+      priceMismatch,
+    };
+  }
+
+  return NextResponse.json({ results });
 }
diff --git a/src/lib/server/models/StockMovement.ts b/src/lib/server/models/StockMovement.ts
new file mode 100644
index 0000000..52b3977
--- /dev/null
+++ b/src/lib/server/models/StockMovement.ts
@@ -0,0 +1,51 @@
+import { Schema, model, models, Model, Types, Document } from "mongoose";
+
+export const MOVEMENT_TYPES = ["SALE", "PURCHASE", "TRANSFER_OUT", "TRANSFER_IN", "ADJUSTMENT"] as const;
+export type MovementType = (typeof MOVEMENT_TYPES)[number];
+
+export interface IStockMovement extends Document {
+  _id: Types.ObjectId;
+  businessId: Types.ObjectId;
+  branchId: Types.ObjectId;
+  productId: Types.ObjectId;
+  type: MovementType;
+  quantity: number; // signed: + adds stock to the branch, - removes it
+  refId: string; // the document that caused it (transaction, purchase, transfer, adjustment)
+  note: string | null;
+  recordedBy: Types.ObjectId | null;
+  createdAt: Date; // authoritative — server clock
+}
+
+/**
+ * The stock ledger. A branch's stock of a product is the SUM of its
+ * movements — there is no separately stored balance that could drift out
+ * of step with history.
+ *
+ * Every movement names the document that caused it, and (type, refId) is
+ * unique. Writing a movement is therefore idempotent: replaying an offline
+ * sale, retrying a transfer, or a second M-Pesa callback can never deduct
+ * or add the same stock twice. A transfer writes two movements with the
+ * same refId but different types (TRANSFER_OUT, TRANSFER_IN).
+ */
+const stockMovementSchema = new Schema<IStockMovement>(
+  {
+    businessId: { type: Schema.Types.ObjectId, ref: "Business", required: true },
+    branchId: { type: Schema.Types.ObjectId, ref: "Branch", required: true },
+    productId: { type: Schema.Types.ObjectId, ref: "Product", required: true },
+    type: { type: String, enum: MOVEMENT_TYPES, required: true },
+    quantity: { type: Number, required: true },
+    refId: { type: String, required: true },
+    note: { type: String, default: null, trim: true },
+    recordedBy: { type: Schema.Types.ObjectId, ref: "User", default: null },
+  },
+  { timestamps: { createdAt: true, updatedAt: false } }
+);
+
+stockMovementSchema.index({ type: 1, refId: 1 }, { unique: true });
+// Stock-level aggregation and the per-branch movement history.
+stockMovementSchema.index({ businessId: 1, branchId: 1, productId: 1 });
+stockMovementSchema.index({ businessId: 1, branchId: 1, createdAt: -1 });
+
+export const StockMovement =
+  (models.StockMovement as Model<IStockMovement>) ||
+  model<IStockMovement>("StockMovement", stockMovementSchema);
diff --git a/src/lib/server/models/StockTransfer.ts b/src/lib/server/models/StockTransfer.ts
new file mode 100644
index 0000000..14856a2
--- /dev/null
+++ b/src/lib/server/models/StockTransfer.ts
@@ -0,0 +1,40 @@
+import { Schema, model, models, Model, Types, Document } from "mongoose";
+
+export interface IStockTransfer extends Document {
+  _id: Types.ObjectId;
+  businessId: Types.ObjectId;
+  fromBranchId: Types.ObjectId;
+  toBranchId: Types.ObjectId;
+  productId: Types.ObjectId;
+  quantity: number;
+  clientUuid: string; // idempotency key from the form, so a double-submit moves stock once
+  note: string | null;
+  recordedBy: Types.ObjectId;
+  createdAt: Date;
+}
+
+/**
+ * An inter-branch stock transfer. The stock itself moves through two
+ * StockMovement rows (TRANSFER_OUT at the source, TRANSFER_IN at the
+ * destination) whose refId is this transfer's id.
+ */
+const stockTransferSchema = new Schema<IStockTransfer>(
+  {
+    businessId: { type: Schema.Types.ObjectId, ref: "Business", required: true },
+    fromBranchId: { type: Schema.Types.ObjectId, ref: "Branch", required: true },
+    toBranchId: { type: Schema.Types.ObjectId, ref: "Branch", required: true },
+    productId: { type: Schema.Types.ObjectId, ref: "Product", required: true },
+    quantity: { type: Number, required: true, min: 1 },
+    clientUuid: { type: String, required: true },
+    note: { type: String, default: null, trim: true },
+    recordedBy: { type: Schema.Types.ObjectId, ref: "User", required: true },
+  },
+  { timestamps: { createdAt: true, updatedAt: false } }
+);
+
+stockTransferSchema.index({ businessId: 1, clientUuid: 1 }, { unique: true });
+stockTransferSchema.index({ businessId: 1, createdAt: -1 });
+
+export const StockTransfer =
+  (models.StockTransfer as Model<IStockTransfer>) ||
+  model<IStockTransfer>("StockTransfer", stockTransferSchema);
diff --git a/src/lib/server/models/Transaction.ts b/src/lib/server/models/Transaction.ts
index 9b02ac8..ae15738 100644
--- a/src/lib/server/models/Transaction.ts
+++ b/src/lib/server/models/Transaction.ts
@@ -9,7 +9,7 @@ export interface ITransaction extends Document {
   branchId: Types.ObjectId;
   staffId: Types.ObjectId;
   customerName: string | null;
-  customerPhone: string;
+  customerPhone: string | null; // optional for cash sales; required for M-Pesa
   productId: Types.ObjectId;
   sizeLiters: number; // snapshotted at sale time
   quantity: number;
@@ -26,8 +26,15 @@ export interface ITransaction extends Document {
     resultDesc: string | null;
   };
 
-  clientUuid: string; // idempotency key generated on-device
+  clientUuid: string; // idempotency key for this line: `${saleId}:${lineIndex}` for cash sales
+  saleId: string | null; // groups the lines of one multi-item sale (the device's sale UUID)
+  lineIndex: number;
   syncedFromOffline: boolean;
+  // What the device showed and collected per unit. The server price stays
+  // authoritative; a difference (e.g. the price changed while the device
+  // was offline) is kept and flagged rather than silently overwritten.
+  clientUnitPrice: number | null;
+  priceMismatch: boolean;
 
   // TAMPER PREVENTION: this is what the device *claimed* the time was when
   // the sale happened — informational only, e.g. for offline-queue
@@ -50,7 +57,7 @@ const transactionSchema = new Schema<ITransaction>(
     branchId: { type: Schema.Types.ObjectId, ref: "Branch", required: true },
     staffId: { type: Schema.Types.ObjectId, ref: "User", required: true },
     customerName: { type: String, default: null, trim: true },
-    customerPhone: { type: String, required: true },
+    customerPhone: { type: String, default: null },
     productId: { type: Schema.Types.ObjectId, ref: "Product", required: true },
     sizeLiters: { type: Number, required: true },
     quantity: { type: Number, required: true, min: 1 },
@@ -73,7 +80,11 @@ const transactionSchema = new Schema<ITransaction>(
     },
 
     clientUuid: { type: String, required: true },
+    saleId: { type: String, default: null },
+    lineIndex: { type: Number, default: 0 },
     syncedFromOffline: { type: Boolean, default: false },
+    clientUnitPrice: { type: Number, default: null },
+    priceMismatch: { type: Boolean, default: false },
     clientSubmittedAt: { type: Date, default: null },
   },
   { timestamps: true } // createdAt/updatedAt set from the server's Date.now(), not client input
@@ -92,6 +103,7 @@ transactionSchema.index({ clientUuid: 1 }, { unique: true });
 transactionSchema.index({ branchId: 1, createdAt: -1 });
 transactionSchema.index({ businessId: 1, createdAt: -1 });
 transactionSchema.index({ paymentStatus: 1 });
+transactionSchema.index({ businessId: 1, saleId: 1 });
 
 export const Transaction =
   (models.Transaction as Model<ITransaction>) ||
diff --git a/src/lib/server/stock.ts b/src/lib/server/stock.ts
new file mode 100644
index 0000000..71418a4
--- /dev/null
+++ b/src/lib/server/stock.ts
@@ -0,0 +1,140 @@
+import mongoose, { Types } from "mongoose";
+import { StockMovement, type MovementType } from "@/lib/server/models/StockMovement";
+import { Branch } from "@/lib/server/models/Branch";
+import { Product } from "@/lib/server/models/Product";
+import type { ITransaction } from "@/lib/server/models/Transaction";
+
+type Id = Types.ObjectId | string;
+
+export interface MovementInput {
+  businessId: Id;
+  branchId: Id;
+  productId: Id;
+  type: MovementType;
+  quantity: number; // signed
+  refId: string;
+  note?: string | null;
+  recordedBy?: Id | null;
+}
+
+/**
+ * Write one ledger movement, idempotently: if a movement with the same
+ * (type, refId) already exists nothing changes. Safe to call again on any
+ * retry or replay.
+ */
+export async function recordMovement(m: MovementInput): Promise<void> {
+  await StockMovement.updateOne(
+    { type: m.type, refId: m.refId },
+    {
+      $setOnInsert: {
+        businessId: m.businessId,
+        branchId: m.branchId,
+        productId: m.productId,
+        quantity: m.quantity,
+        note: m.note ?? null,
+        recordedBy: m.recordedBy ?? null,
+      },
+    },
+    { upsert: true }
+  );
+}
+
+/**
+ * Deduct a completed sale from its branch's stock. Only SUCCESS sales move
+ * stock — a PENDING or FAILED M-Pesa prompt never does.
+ */
+export async function recordSaleMovement(
+  txn: Pick<ITransaction, "_id" | "businessId" | "branchId" | "productId" | "quantity" | "paymentStatus" | "staffId">
+): Promise<void> {
+  if (txn.paymentStatus !== "SUCCESS") return;
+  await recordMovement({
+    businessId: txn.businessId,
+    branchId: txn.branchId,
+    productId: txn.productId,
+    type: "SALE",
+    quantity: -Math.abs(txn.quantity),
+    refId: txn._id.toString(),
+    recordedBy: txn.staffId,
+  });
+}
+
+/** Ledger refIds derived from client-generated keys are namespaced by tenant. */
+export function clientRefId(businessId: Id, clientUuid: string): string {
+  return `${businessId.toString()}:${clientUuid}`;
+}
+
+/** Current stock of one product at one branch (the sum of its movements). */
+export async function stockOf(businessId: Id, branchId: Id, productId: Id): Promise<number> {
+  const [row] = await StockMovement.aggregate<{ total: number }>([
+    {
+      $match: {
+        businessId: new mongoose.Types.ObjectId(businessId.toString()),
+        branchId: new mongoose.Types.ObjectId(branchId.toString()),
+        productId: new mongoose.Types.ObjectId(productId.toString()),
+      },
+    },
+    { $group: { _id: null, total: { $sum: "$quantity" } } },
+  ]);
+  return row?.total ?? 0;
+}
+
+export type StockStatus = "OK" | "LOW" | "OUT";
+
+export interface StockLevel {
+  branchId: string;
+  branchName: string;
+  productId: string;
+  productLabel: string;
+  unit: string;
+  quantity: number;
+  lowStockThreshold: number;
+  status: StockStatus;
+}
+
+export function stockStatus(quantity: number, threshold: number): StockStatus {
+  if (quantity <= 0) return "OUT";
+  if (quantity <= threshold) return "LOW";
+  return "OK";
+}
+
+/**
+ * Stock of every active product at every branch in scope — including
+ * zero rows, so an owner sees "0" rather than a missing product. A
+ * negative quantity is reported as-is: it means sales were recorded
+ * (typically offline) for stock the ledger never received, and the branch
+ * needs a count/adjustment.
+ */
+export async function getStockLevels(businessId: Id, branchId?: Id | null): Promise<StockLevel[]> {
+  const bizObj = new mongoose.Types.ObjectId(businessId.toString());
+  const branchFilter: Record<string, unknown> = { businessId: bizObj };
+  if (branchId) branchFilter._id = new mongoose.Types.ObjectId(branchId.toString());
+
+  const [branches, products, totals] = await Promise.all([
+    Branch.find(branchFilter).sort({ branchName: 1 }).lean(),
+    Product.find({ businessId: bizObj, isActive: true }).sort({ sortOrder: 1 }).lean(),
+    StockMovement.aggregate<{ _id: { branchId: Types.ObjectId; productId: Types.ObjectId }; total: number }>([
+      { $match: branchId ? { businessId: bizObj, branchId: branchFilter._id } : { businessId: bizObj } },
+      { $group: { _id: { branchId: "$branchId", productId: "$productId" }, total: { $sum: "$quantity" } } },
+    ]),
+  ]);
+
+  const totalByKey = new Map(totals.map((t) => [`${t._id.branchId}:${t._id.productId}`, t.total]));
+  const levels: StockLevel[] = [];
+  for (const b of branches) {
+    for (const p of products) {
+      const quantity = totalByKey.get(`${b._id}:${p._id}`) ?? 0;
+      const lowStockThreshold = p.lowStockThreshold ?? 10;
+      levels.push({
+        branchId: b._id.toString(),
+        branchName: b.branchName,
+        productId: p._id.toString(),
+        productLabel: p.label,
+        unit: p.unit ?? "L",
+        quantity,
+        lowStockThreshold,
+        status: stockStatus(quantity, lowStockThreshold),
+      });
+    }
+  }
+  return levels;
+}
diff --git a/src/lib/syncProtocol.ts b/src/lib/syncProtocol.ts
new file mode 100644
index 0000000..5f1a5e8
--- /dev/null
+++ b/src/lib/syncProtocol.ts
@@ -0,0 +1,11 @@
+// Shared between the terminal's offline queue and POST /api/transactions/cash.
+
+/** Largest batch the offline queue sends in one request. */
+export const MAX_SALES_PER_REQUEST = 50;
+
+/** Largest number of product lines in one sale. */
+export const MAX_LINES_PER_SALE = 20;
+
+export type SaleResult =
+  | { clientUuid: string; status: "saved" | "duplicate"; amountTotal: number; priceMismatch: boolean }
+  | { clientUuid: string; status: "rejected"; error: string };
diff --git a/tests/stock.test.ts b/tests/stock.test.ts
new file mode 100644
index 0000000..fde6fde
--- /dev/null
+++ b/tests/stock.test.ts
@@ -0,0 +1,134 @@
+import { describe, expect, it } from "vitest";
+import { stockOf } from "@/lib/server/stock";
+import * as stock from "@/app/api/stock/route";
+import * as adjustments from "@/app/api/stock/adjustments/route";
+import * as transfers from "@/app/api/stock/transfers/route";
+import * as purchases from "@/app/api/purchases/route";
+import { json, makeTenant, req, uuid, type Actor, type Tenant } from "./helpers";
+
+async function addStock(t: Tenant, as: Actor, branchId: string, productId: string, quantity: number) {
+  const res = await adjustments.POST(
+    req("POST", "/x", { as, body: { clientUuid: uuid(), branchId, productId, mode: "DELTA", quantity, note: "opening" } })
+  );
+  expect(res.status).toBe(201);
+}
+
+describe("stock ledger", () => {
+  it("adds purchased stock to the receiving branch", async () => {
+    const t = await makeTenant();
+    const res = await purchases.POST(
+      req("POST", "/x", {
+        as: t.managerA,
+        body: { supplierName: "Kiwanja Water", productId: t.water20, quantity: 40, unitCost: 120 },
+      })
+    );
+    expect(res.status).toBe(201);
+    expect(await stockOf(t.businessId, t.branchA, t.water20)).toBe(40);
+  });
+
+  it("corrects stock to a physical count, once per submit", async () => {
+    const t = await makeTenant();
+    await addStock(t, t.owner, t.branchA, t.water20, 50);
+    const body = { clientUuid: uuid(), productId: t.water20, mode: "COUNT", quantity: 47, note: "evening count" };
+    const first = await json(await adjustments.POST(req("POST", "/x", { as: t.managerA, body })));
+    expect(first.body.quantity).toBe(47);
+    // A double-submit of the same count changes nothing further.
+    const again = await json(await adjustments.POST(req("POST", "/x", { as: t.managerA, body })));
+    expect(again.body.quantity).toBe(47);
+  });
+
+  it("does not let a manager adjust another branch", async () => {
+    const t = await makeTenant();
+    const res = await adjustments.POST(
+      req("POST", "/x", {
+        as: t.managerA,
+        body: { clientUuid: uuid(), branchId: t.branchB, productId: t.water20, mode: "DELTA", quantity: 5 },
+      })
+    );
+    expect(res.status).toBe(403);
+  });
+
+  it("reports stock per branch with low and out-of-stock flags", async () => {
+    const t = await makeTenant();
+    await addStock(t, t.owner, t.branchA, t.water20, 8); // threshold defaults to 10
+    await addStock(t, t.owner, t.branchA, t.water5, 30);
+    const { body } = await json(await stock.GET(req("GET", "/api/stock", { as: t.owner })));
+    const find = (b: string, p: string) => body.levels.find((l: any) => l.branchId === b && l.productId === p);
+    expect(body.levels).toHaveLength(4); // 2 branches x 2 products, zero rows included
+    expect(find(t.branchA, t.water20)).toMatchObject({ quantity: 8, status: "LOW" });
+    expect(find(t.branchA, t.water5)).toMatchObject({ quantity: 30, status: "OK" });
+    expect(find(t.branchB, t.water20)).toMatchObject({ quantity: 0, status: "OUT" });
+  });
+
+  it("shows a cashier only their own branch's stock", async () => {
+    const t = await makeTenant();
+    const { body } = await json(await stock.GET(req("GET", "/api/stock?branchId=" + t.branchB, { as: t.cashierA })));
+    expect(new Set(body.levels.map((l: any) => l.branchId))).toEqual(new Set([t.branchA]));
+  });
+});
+
+describe("inter-branch transfers", () => {
+  it("moves stock out of one branch and into the other", async () => {
+    const t = await makeTenant();
+    await addStock(t, t.owner, t.branchA, t.water20, 30);
+    const res = await transfers.POST(
+      req("POST", "/x", {
+        as: t.owner,
+        body: { clientUuid: uuid(), fromBranchId: t.branchA, toBranchId: t.branchB, productId: t.water20, quantity: 12 },
+      })
+    );
+    expect(res.status).toBe(201);
+    expect(await stockOf(t.businessId, t.branchA, t.water20)).toBe(18);
+    expect(await stockOf(t.businessId, t.branchB, t.water20)).toBe(12);
+  });
+
+  it("applies a double-submitted transfer once", async () => {
+    const t = await makeTenant();
+    await addStock(t, t.owner, t.branchA, t.water20, 30);
+    const body = { clientUuid: uuid(), toBranchId: t.branchB, productId: t.water20, quantity: 10 };
+    await transfers.POST(req("POST", "/x", { as: t.managerA, body }));
+    const second = await transfers.POST(req("POST", "/x", { as: t.managerA, body }));
+    expect(second.status).toBe(200);
+    expect(await stockOf(t.businessId, t.branchA, t.water20)).toBe(20);
+    expect(await stockOf(t.businessId, t.branchB, t.water20)).toBe(10);
+  });
+
+  it("refuses to send more than the branch has", async () => {
+    const t = await makeTenant();
+    await addStock(t, t.owner, t.branchA, t.water20, 5);
+    const res = await transfers.POST(
+      req("POST", "/x", {
+        as: t.managerA,
+        body: { clientUuid: uuid(), toBranchId: t.branchB, productId: t.water20, quantity: 6 },
+      })
+    );
+    expect(res.status).toBe(409);
+    expect(await stockOf(t.businessId, t.branchA, t.water20)).toBe(5);
+  });
+
+  it("only lets a manager send from their own branch", async () => {
+    const t = await makeTenant();
+    await addStock(t, t.owner, t.branchB, t.water20, 5);
+    const res = await transfers.POST(
+      req("POST", "/x", {
+        as: t.managerA,
+        body: { clientUuid: uuid(), fromBranchId: t.branchB, toBranchId: t.branchA, productId: t.water20, quantity: 1 },
+      })
+    );
+    expect(res.status).toBe(403);
+  });
+
+  it("cannot move stock into another tenant's branch", async () => {
+    const t = await makeTenant("Alpha");
+    const other = await makeTenant("Beta");
+    await addStock(t, t.owner, t.branchA, t.water20, 5);
+    const res = await transfers.POST(
+      req("POST", "/x", {
+        as: t.owner,
+        body: { clientUuid: uuid(), fromBranchId: t.branchA, toBranchId: other.branchA, productId: t.water20, quantity: 1 },
+      })
+    );
+    expect(res.status).toBe(404);
+    expect(await stockOf(t.businessId, t.branchA, t.water20)).toBe(5);
+  });
+});
diff --git a/tests/sync.test.ts b/tests/sync.test.ts
new file mode 100644
index 0000000..dc0c19b
--- /dev/null
+++ b/tests/sync.test.ts
@@ -0,0 +1,129 @@
+import { describe, expect, it } from "vitest";
+import { Transaction } from "@/lib/server/models/Transaction";
+import { Product, BranchPricing } from "@/lib/server/models/Product";
+import { stockOf } from "@/lib/server/stock";
+import * as cash from "@/app/api/transactions/cash/route";
+import type { SaleResult } from "@/lib/syncProtocol";
+import { json, makeTenant, req, uuid, type Actor, type Tenant } from "./helpers";
+
+function sale(t: Tenant, cashier: Actor, lines: { productId: string; quantity: number; unitPrice?: number }[], extra = {}) {
+  return {
+    clientUuid: uuid(),
+    staffId: cashier.id,
+    branchId: cashier.branchId!,
+    clientSubmittedAt: new Date().toISOString(),
+    lines,
+    ...extra,
+  };
+}
+
+async function post(as: Actor, sales: unknown[]) {
+  return json<{ results: SaleResult[] }>(await cash.POST(req("POST", "/api/transactions/cash", { as, body: { sales } })));
+}
+
+describe("offline sale sync", () => {
+  it("records a multi-item sale as one line per product and deducts each from stock", async () => {
+    const t = await makeTenant();
+    const s = sale(t, t.cashierA, [
+      { productId: t.water20, quantity: 2, unitPrice: 200 },
+      { productId: t.water5, quantity: 3, unitPrice: 60 },
+    ]);
+    const { status, body } = await post(t.cashierA, [s]);
+    expect(status).toBe(200);
+    expect(body.results).toEqual([{ clientUuid: s.clientUuid, status: "saved", amountTotal: 580, priceMismatch: false }]);
+    expect(await Transaction.countDocuments({ saleId: s.clientUuid })).toBe(2);
+    expect(await stockOf(t.businessId, t.branchA, t.water20)).toBe(-2);
+    expect(await stockOf(t.businessId, t.branchA, t.water5)).toBe(-3);
+  });
+
+  it("reports a replayed sale as duplicate, so the device can stop resending it", async () => {
+    const t = await makeTenant();
+    const s = sale(t, t.cashierA, [{ productId: t.water20, quantity: 1 }]);
+    await post(t.cashierA, [s]);
+    const replay = await post(t.cashierA, [s]);
+    expect(replay.body.results[0]).toMatchObject({ clientUuid: s.clientUuid, status: "duplicate" });
+    expect(await Transaction.countDocuments({ saleId: s.clientUuid })).toBe(1);
+    expect(await stockOf(t.businessId, t.branchA, t.water20)).toBe(-1);
+  });
+
+  it("syncs a 120-sale backlog in chunks with no duplicates (the old 100-row dead end)", async () => {
+    const t = await makeTenant();
+    const backlog = Array.from({ length: 120 }, () => sale(t, t.cashierA, [{ productId: t.water20, quantity: 1 }]));
+    // An over-sized batch is refused outright...
+    const tooBig = await cash.POST(req("POST", "/x", { as: t.cashierA, body: { sales: backlog } }));
+    expect(tooBig.status).toBe(400);
+    // ...and the device sends it in chunks of 50, twice over (a full replay).
+    for (let pass = 0; pass < 2; pass++) {
+      for (let i = 0; i < backlog.length; i += 50) {
+        const { status } = await post(t.cashierA, backlog.slice(i, i + 50));
+        expect(status).toBe(200);
+      }
+    }
+    expect(await Transaction.countDocuments({ businessId: t.businessId })).toBe(120);
+    expect(await stockOf(t.businessId, t.branchA, t.water20)).toBe(-120);
+  });
+
+  it("rejects one bad sale without blocking the rest of the batch", async () => {
+    const t = await makeTenant();
+    const good = sale(t, t.cashierA, [{ productId: t.water20, quantity: 1 }]);
+    const malformed = { clientUuid: uuid(), lines: [] };
+    const { body } = await post(t.cashierA, [malformed, good]);
+    const byId = Object.fromEntries(body.results.map((r) => [r.clientUuid, r.status]));
+    expect(byId[malformed.clientUuid]).toBe("rejected");
+    expect(byId[good.clientUuid]).toBe("saved");
+  });
+
+  it("refuses to credit a queued sale to whoever happens to be signed in", async () => {
+    const t = await makeTenant();
+    const otherCashiersSale = sale(t, t.cashierB, [{ productId: t.water20, quantity: 1 }]);
+    const { body } = await post(t.cashierA, [otherCashiersSale]);
+    expect(body.results[0].status).toBe("rejected");
+    expect(await Transaction.countDocuments({})).toBe(0);
+  });
+
+  it("keeps a sale at the branch where it happened after the cashier is moved", async () => {
+    const t = await makeTenant();
+    const s = sale(t, t.cashierA, [{ productId: t.water20, quantity: 1 }]); // recorded at branch A
+    const moved = { ...t.cashierA, branchId: t.branchB };
+    await post(moved, [s]);
+    const txn = await Transaction.findOne({ saleId: s.clientUuid });
+    expect(txn!.branchId.toString()).toBe(t.branchA);
+  });
+
+  it("rejects a sale naming another tenant's product or branch", async () => {
+    const t = await makeTenant("Alpha");
+    const other = await makeTenant("Beta");
+    const foreignProduct = sale(t, t.cashierA, [{ productId: other.water20, quantity: 1 }]);
+    const foreignBranch = sale(t, t.cashierA, [{ productId: t.water20, quantity: 1 }], { branchId: other.branchA });
+    const { body } = await post(t.cashierA, [foreignProduct, foreignBranch]);
+    expect(body.results.map((r) => r.status)).toEqual(["rejected", "rejected"]);
+    expect(await Transaction.countDocuments({})).toBe(0);
+  });
+
+  it("uses the branch price, flags a price the device showed differently, and keeps deactivated products sellable", async () => {
+    const t = await makeTenant();
+    await BranchPricing.create({ businessId: t.businessId, branchId: t.branchA, productId: t.water20, unitPrice: 180 });
+    await Product.updateOne({ _id: t.water5 }, { isActive: false });
+    const s = sale(t, t.cashierA, [
+      { productId: t.water20, quantity: 1, unitPrice: 200 }, // stale price on the device
+      { productId: t.water5, quantity: 1, unitPrice: 60 },
+    ]);
+    const { body } = await post(t.cashierA, [s]);
+    expect(body.results[0]).toMatchObject({ status: "saved", amountTotal: 240, priceMismatch: true });
+  });
+
+  it("accepts a cash sale with no customer phone, and keeps one with a bad phone", async () => {
+    const t = await makeTenant();
+    const noPhone = sale(t, t.cashierA, [{ productId: t.water20, quantity: 1 }]);
+    const badPhone = sale(t, t.cashierA, [{ productId: t.water20, quantity: 1 }], { customerPhone: "12" });
+    const { body } = await post(t.cashierA, [noPhone, badPhone]);
+    expect(body.results.map((r) => r.status)).toEqual(["saved", "saved"]);
+  });
+
+  it("still accepts the old single-product payload", async () => {
+    const t = await makeTenant();
+    const legacy = { clientUuid: uuid(), customerPhone: "0712345678", productId: t.water20, quantity: 2 };
+    const { body } = await post(t.cashierA, [legacy]);
+    expect(body.results[0]).toMatchObject({ status: "saved", amountTotal: 400 });
+  });
+});