fix(pos): clamp payment amount to total + allow 1c tolerance for rounding

project/apps/pos/src/app/(terminal)/page.tsx

@@ -1906,15 +1906,17 @@ export default function RegisterPage() {
                   key={method.id}
                   type="button"
                   onClick={() => {
-                    // POS-SELFPAY-FLOW: no partial payments — directly add full payment and confirm
+                    // POS-SELFPAY-FLOW: no partial payments — directly add full payment and confirm.
+                    // Clamp to totals.total to avoid 1-cent rounding mismatches with the backend.
                     setShowSelfpayPaymentMethods(false);
+                    const amountCents = Math.min(remainingCents, totals.total);
                     const allocation: PaymentAllocation = {
                       id: generateIdempotencyKey(),
                       methodCode: method.code,
                       methodLabel: method.label,
                       kind: method.kind,
-                      amountCents: remainingCents,
-                      ...(method.kind === 'cash' ? { tenderedCents: remainingCents } : {}),
+                      amountCents,
+                      ...(method.kind === 'cash' ? { tenderedCents: amountCents } : {}),
                       changeCents: 0,
                     };
                     void confirmSaleWith([allocation]);

project/src/modules/pos/application/create-pos-sale.ts

@@ -111,7 +111,10 @@ export function validatePaymentAllocations(
     };
   });
   const allocatedCents = validated.reduce((sum, payment) => sum + payment.amountCents, 0);
-  if (allocatedCents > totalCents) {
+  // Allow up to 1 cent tolerance for rounding differences between the front-end
+  // POS calculation (which uses cached prices) and the back-end canonical total
+  // (which re-reads the catalogue under a transaction).
+  if (allocatedCents - totalCents > 1) {
     throw new AppError(
       400,
       'POS_PAYMENT_OVERPAYMENT',