NOTE: This article walks you though the process in classic ASAP. For instructions in the latest version of ASAP, see Apply account credit.
To apply an ASAP credit on an invoice, you would need to actually go under the "Actions" section to do so. Once done, you can select to "Apply Credit" and a dialogue will pop up that needs confirmation in order to do so:
Selecting this option will bring a pop-up option, allowing you to enter in the amount of credit you would like to apply as well as any notes you would like to be displayed under the Invoice Transaction Details. Once you've selected "Save Changes", the invoice will be updated to reflect the transaction. You can only enter in up to the amount of credit found in the customer's account and not more than what is actually owed on the invoice.