You can delete the invoice payment to make an invoice unpaid. Please follow these steps:
Open the invoice.
Click the 1 payment made hyperlink in under the PAID status.
Click the payment date.
On the Receive Payment page, click More.
Choose Delete.
If you have other questions, you can always go back to this thread.
You are leaving Intuit.
You have clicked a link to a site outside of the QuickBooks or ProFile Communities. By clicking "Continue", you will leave the community and be taken to that site instead.