We can get the original invoice back to an Unpaid status after marking it as paid by deleting the payment associated with it, sales319.

To proceed, let’s open the invoice to unlink the payment.

Follow the steps below:

1. Navigate to Sales and click the Invoices from the left-hand menu.
2. Click on View/Edit under the Action column.
3. Locate and click the Payment Link on the invoice page.
   
   
4. In the Receive Payment window, uncheck the invoice and any applied credits, or click Delete from the More options.
5. Select the Delete button and confirm on the next window.
   
   

If you mean something else, please let me know by adding a reply below.