Hello, @debi8.

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. 

Since you’ve already received the payment from your customer, what you can do is delete the payment that you received so you can apply the credit memo to the invoice.

Here’s how.

1. Go to the Sales menu.
2. Select Customer.
3. Find the customer that you wanted to apply for the credit memo.
4. Click the Payment of the customer.
5. On the Receive Payment window, click More.
6. Then, select Delete.

Please refer to the screenshots below.

Once done, apply your credit memo to the invoice.

1. Click Sales from the left pane.
2. Select Customers and click the customer’s name.
3. Locate the invoice you want to apply the credit to and click Receive Payment.
4. Enter the necessary information on the Receive Payment window.
5. In the Outstanding Transactions section, make sure that the correct invoice is selected.
6. Click Save and close.

For your reference you may check out this article about credit memo, credit, or refund. Enter and apply credit memos and delayed credits in QuickBooks Online.

Please let me know if you have any questions. I’ll be here to help. Take care and keep safe.