Deleting a Supplier Payment

I have a supplier invoice that was created from a purchase order. A payment was somehow entered by accident for the full amount. The operator then turned this into an invoice. The payment is wrong and I want to reverse it. 

I can't delete the payment because the invoice was generated from a PO.

I can't delete the invoice for the same reason.

I can't enter a credit for the invoice as the fully paid invoice is not showing. (This is the option advised in the help topic).

I can't delete the remittance advice. Originally there was none generated, so I did this thinking I could delete it.

Should I enter a negative payment against the invoice?

Is there another way around this?