Invoice written off but still showing amounts outstanding

I have a sales invoice in US Dollars that the customer paid but it was US$20 short due to deduction of international bank charges en route. So on the invoice page it says outstanding $20, but it also says -£153.13 (which I take it is a FX adjustment) - see attached screenshot (sorry about the triplicate!)

But somewhere in the past I must have tried to even it out by writing the balance off.

The amount in GBP is showing as a negative in my debtors report.

So now I want to issue a credit for the $20 to reduce USD outstanding to nil, and then do an FX adjustment to clear off the GBP. I have done this on similar invoices so far (trying to tidy up year end). But it won't let me do anything on this invoice as status is - written off.

Any ideas?

If I delete the payment attached to the invoice will this un-write-off the invoice? Or cause more problems trying to re-explain the payment to a written-off invoice?