How do I move a member's overpayment of a membership dues invoice to his wife's membership dues invoice?

Hello there, @fitzmiller! I'll help transfer the invoice credit.

In QuickBooks Online, you can create a journal entry to move an overpayment or credit from one customer to the other. I'll show you how.

1. Go to the +New button.
2. Under Other, select Journal Entry.
3. Select a date for this journal entry.
4. On the Account column, choose your Accounts Receivable account.
5. Below the Debits column, enter the amount of overpayment.
6. Under the Name column, select the name of the customer to whom the credit is transferred from.
7. On the second line item, select your Accounts Receivable account under the Account column.
8. Enter the amount of credit under the Credit column.
9. Below the Name column on the second line item, choose the name of the customer to whom the overpayment is transferred.
10. Click Save and close.

Once done, apply the credit to the invoice in question accordingly. Here's an article you can read to learn more about transferring the amount of a customer's credit: How to Transfer a Credit from One Customer to Another in QuickBooks Online?

I've also included this helpful reference for the steps in creating a document showing all your customer sales activity and outstanding transactions: How to Send Statements to Show Customers Summaries in QuickBooks Online?

Don't hesitate to post a reply below if you have other questions or concerns with QuickBooks payments and credits. I'm always around happy to help. Stay safe and have a good one!

"apply the credit to the invoice in question accordingly" and how do I do that? Picking the other suggestions do not work either.

Joining this thread, fitzmiller.  Allow me to help you apply a credit memo to your customer’s invoice in QuickBooks Online.

If a customer paid you more than what was owed, returning a product, requesting a refund you can create a Credit Memo. Once done, you can apply it to an open invoice for a given customer. 

Step 1: Enter the credit memo:

1. Select + New.
2. Select Credit memo.
3. Fill out all required fields (including the customer name, date, products/service items, quantities, and amounts).
4. Select Save and close.

Step 2: Apply the credit memo:

1. Select + New.
2. Select Receive payment.
3. Complete these fields as follows:
   • Customer Note: Make sure you choose the customer or project you selected in the invoice.
   • Date
4. Under Credits, select Credit memo.
5. Verify that the total to pay (if any) is correct after applying the Credit Memo.
6. Select Save and close.

I'd recommend checking out this article: Create and apply credit memos or delayed credits in QuickBooks Online. 

If your customer wants a refund, you can refer to this article for the complete guide: Record a customer refund in QuickBooks Online.

It would be my pleasure helping you further if you have additional concerns about invoices. Take care and have a great day!