Hi there, @coopdigs2021.

Thanks for posting here. I can share steps to help you fix the balance of your customer. To start, let's first create a backup copy of your file. This is to ensure that you have original data that you can restore at any time in case of data loss.

Once done, let's now click the zero amount on the customer's balance to see a piece of detailed information. From there, you can either see payments or journal entry which was not applied to the invoice. You have 2 options you can perform.

First option:

1. You'll have to open the unpaid invoice.
2. Double-click to open.
3. At the bottom part, click the Apply credits button.
4. Select available credits.
5. Then, Done.

Second option:

1. Open the payment received.
2. Then, apply the payment to an open invoice.
3. Click Save and close when done.

For more info about the process, please see this article: Give your customer a credit or refund in QuickBooks Desktop for Windows.

Also, you can always seek self-help articles on our Community page to help you finish your QuickBooks tasks in no time.

Don't hesitate to post here again if you have further concerns about fixing your customer's balance. I'll be around to help. Keep safe.