A credit can be kept in a patron account and used later to pay bills.
To create or add a credit, on the Bills screen, select the payment method, type in the amount, select Convert Change to Patron Credit box at the bottom, then click Apply Payment.
Change from a payment can also be converted to credit. In the screenshot below $10.00 has been received for a $4.00 bill with $6.00 applied to patron credit instead of change returned.