I understand you want to un-apply the credit memo added to the wrong invoice. I'm happy to share insights to ensure you'll be able to add the credit memo to the correct invoice.
To correct this, you need to delete the payment for the invoice where you added the credit memo. Once done, you'd want to recreate the invoice payment and make sure to apply the credit to the correct invoice.
Click the Customers tab, then choose Receive Payments.
From the Customer: Job ▼ dropdown, select the customer's name.
I was able to solve the problem, but I do have one more question.
This was not actually a credit memo. It was an overpayment on another invoice.
The difference is that this kind of overpayment does not show up in the list of credit memos I can get in the Transaction Center. Is there any way to see these overpayments itemized, and see how they were applied?