Hi Chippy Coates
Kevin has it spot on but it looks like you've used the "Allocate" button on the invoice which takes you to the "unallocated cash" system, designed primarily for payments in advance. This is actually the method suggested in CB's guide (link below) but is really fiddly and involves you creating a dummy invoice for the foreign exchange profit and thus allocating the payment against both.
Far easier to either explain the payment from the imported bank statement or, if you can't wait for that, creating your own one line bank statement by using the "Money in" option next to the bank account. Either will enable you to use Kevin's method above.
Whilst I'm on, if you ever get a FOREX loss, you can use the same method but you enter a negative sum for the exchange loss first, in the transactions box, which will then provide the sum needed to pay off the invoice.