May I ask if you have encountered this issues?
Thanks in advance
Only time I've ever seen this message is when the Exchange Rate on the Document is different to the Exchange Rate on the Payment. It posts the application and the Realised Gain/loss at the same time. BC (and also NAV) can't just reverse this transaction for some reason when you want to reverse the entry but it will reverse the Gain/loss if you unapply the entries first. After that you can post new entries and apply the correct ones together.
Paul this issue is because you might have done a Forex transaction and the Currency Exchange Rate must have changed.
So Business Central will automatically post this entry to match with your Forex Currency and Local Currency as per the Exchange Rate.
What you can do is unapply this entry with the reconciled transaction and settle this payment by creating a Purchase Journal and settle with the current Payment.
Or you can create a reversing entry using General Journals and settle the Payment with this General Journal Entry.