When Cash is selected, the payment screen has a field to enter the value received from the customer; the terminal live-computes Change = amount entered − order total (never negative) and displays it as "X.XX CHANGE" — or "NO CHANGE" if the entered amount doesn't exceed the total — before you tap Tender to finalize the sale. The Tender button stays disabled until the entered amount is at least the order total. The change amount is also shown on the "Transaction complete" confirmation screen after the sale finishes.