At times, you might have to give a credit to the client as a courtesy or for any other reason. This can be done by writing off all or part of the invoice amount. If you sync Core data with your accounting software, write-off transfers as a credit or discount.
You can generate a write-off from the Payments screen (main menu > Billing) in Core. Check the Core Help for details on write-off.
Another option is to give a project credit to the client using the Credit Memos feature. You can create a credit memo for a project, post it to the relevant account and later pay this liability to the client. Check the Core Help for more on creating a credit memo.