Net off a Purchase (Bill) and Invoice via a Bank Transaction

Hi,

I'm having a bit of a brain freeze and am hoping someone can assist in finding the best way to solve my issue.

We have an Invoice and a Purchase for/from the same supplier, both for different amounts - for simplicity let's say Invoice is £500, Purchase is £400. Both are currently Unpaid.

The supplier has paid the net of the two, so I have a £100 bank payment to explain away... I've tried a number of options but can't seem to find a clean way to flag the £100 payment and mark the Invoice and Purchase as paid. Am I missing something obvious, or is there a sneaky way I need to resolve this? I've done a quick search to see if this question has been asked before but I couldn't see anything obvious...

Thanks in advance for any tips anyone can give :)

Jason