After installing the QuickBooks for Mac 2013 R7 update, users are unable to apply existing credits to an open invoice.
This issue has been resolved in the latest release for QuickBooks for Mac 2013.
Go to the Update to the Latest Release web page.
IMPORTANT: Make sure your product is selected. If not, click the Change link and select your QuickBooks product.
Select and follow the instructions for one of the update options:
Follow the steps below if you want to apply an existing credit to an invoice you created.