Using Multiple Payment Methods
If your customer is paying with more than one type of payment - say, cash in addition to a Gift Card - you will use the Split/Other payment icon:
This will allow you to select a payment method from the menu, enter the amount, and click the Enter button to apply it to the invoice.
Then, select another method from the menu, enter the amount, and click the Enter button to apply it to the invoice again.
Continue until it's paid in full, then click the Complete Sale button.
You may also want to refer to: Create a Customer Account Balance to be Paid Later