If you accidentally pay the wrong invoice or need to remove a payment from an invoice, click on the 'Work' tab. Click on the 'Options' box next to the invoice which you want to remove the payment allocated against it.
Choose the 'View Invoice Summary' option and this will take you to the invoice screen with the payment details at the bottom. Click the 'Options' box on the right hand side next to the payment.
From the options, choose 'Delete Payment'. This will bring up the slider screen asking you to confirm that you wish to delete the payment against the invoice.
Click the blue 'Delete' button and this will remove the payment and mark the invoice as outstanding. Click 'Update Invoice' to save.