Hello, @es_sfo.

I hope you have had a good day. I’m here to get you back on track and help you mark an invoice unpaid. 

Here are the few steps to mark the invoice as unpaid:

1. Pick Sales and then choose Invoices.
2. Locate the invoice that needs the reverse payment.
3. Find the 1 Payment link under the paid stamp.
4. Select the link for the date.
5. Once you come to the Receive Payment, find More at the bottom of the page.
6. Push on Delete and then Press Yes.

Once done, you can apply the credit to the invoice.

Let me guide you:

1. Go to Sales.
2. Find Customers and click the customer’s name.
3. Choose the invoice you want to apply the credit to then hit Receive Payment.
4. Enter in any information that needs to be filled out on the Receive Payment window.
5. Make sure the invoice is selected when you’re in the Outstanding Transaction section.
6. Select Save and Close.

For future reference, visit Enter and Apply Credit Memos and Delayed Credits.

Let me know if there’s anything else I can help with. Take care.