I think you go into Receipt of Invoice and create an invoice for negative 25 at $4.136 ea. Then do another entry in Receipt of Invoice for positive 20 at $5.17 each.

Then you can apply these two invoices to each other in Manual Check Entry.

And after that, change the PO ordered amount to 20 also to close the PO.

Thank you all so much!

An extension to this question is what happens when the quantity received is greater than the quantity ordered?
We have a new person in the warehouse and for some reason this person has been receiving the same order twice (non serial items), maybe the person forgot that he received them the first time. Sage blissfully accepts this and boom! we have double quantities received in the PO, which we identified when we did inventory counts at the end of the month. We corrected this by doing retains.
So how do we avoid this happening again, or at least identify it sooner? The person has been retrained but remains being human.

Role Maintenance:

In some cases you do receive more than ordered, for example you order 4 but the supplier only ships in quantity of 5. I suppose the best practice would be to amend the PO to match the qty shipped by the supplier. This should actually be amended by the person placing the order since they would be notified about the minimum order quantity.... I guess.