Positive amount tops up, negative redeems. Redemptions that would take the balance below zero are rejected; the balance check and decrement are a single atomic operation, so two simultaneous redemptions on the same card can't both succeed and overdraw. UniqueAdjustmentId is an idempotency key: a repeat call with the same value is rejected and should be treated as positive confirmation the first call succeeded. Balance is synced to connected sales channels.