Hi MrMacca! Good question, thanks for posting it!
The payment during reconciliation needs to be adjusted to match the statement entry.
This'll unallocate the client payment from the invoice, so that'll need to be allocated back to it.
Credit note the outstanding amount of the invoice, and this'll then change it from having an outstanding amount to being settled.
Create a dummy bank account (call it whatever you like - 'Transaction Fee's might be sensible!) and then edit the expense item to be marked as being paid out of that dummy bank account.
Then you'll just need to record any transaction fees (that don't appear on your bank statement) as being paid out from this account.
You don't need to reconcile the dummy account, it'll purely be used to record and store a record of any transaction fee's that you've taken from invoices. That way, you can show the losses you make on some of your income because of the fees.
Helpful links:
Hope this clears things up for you!