Credit hiccups ...

Thank you for reaching out with your concern. I understand how challenging it can be to experience issues when applying credits to invoices in QuickBooks Desktop, @Deadwood Al. I'm here to guide you through this process step by step.

The message stating, "Another user has made changes..." indicates that multiple users are accessing the same data file or there's a synchronization delay.

Thus, let's switch to Single-user mode so you can apply the credit successfully.

Here's how: 

1. Open your QuickBooks Desktop. 
2. Go to the File menu located at the top left corner.
3. Choose the Switch to Single-user Mode.
    

If the issue persists, I recommend using the Verify and Rebuild Data tool in QuickBooks Desktop to determine if there's a data issue with your company file that prevents you from applying the credit to the invoice.

Additionally, you can view the following materials to learn more about the limitations of Single-user Mode and Multi-user Mode:  

• Features available and tasks you can perform in a single or multi-user mode in QuickBooks Desktop
• QuickBooks Desktop users and restrictions

I hope these steps help you resolve the credit application issue. Your patience is greatly appreciated. Should further glitches arise, know we are here for you and ready to assist.

To need to switch to single user mode (and I do know how after years of QB work) in order to apply a credit is ludicrous. And I have experienced this over quite some time, with more than a few verify and rebuilds scattered in between, so those make zero difference in what I'm experiencing. Sounds like you really have no clue what is causing the problem, so I assume it is just an "undocumented feature" from the designers of this module.