Amending received goods after invoice in POP- best solution

SOLVED

We have had an increase in customers invoicing POs for the wrong received amounts.

i.e. they incorrectly record that they received 4 of 6 items in their PO in POP, and then invoice for 6. They actually received the 6 but recorded the wrong amount. They then can't amend the received amount as they have invoiced it. What would be the best suggestion for this? The PO just stays live status and won't complete.

There just doesn't seem to be a way to fix this in Sage200 professional. You can't delete the PO as it's been invoiced. You can reverse the invoice, but it won't affect the POP side, only the PL. 

I am just stuck on what the most efficient and correct solution would be in this situation as it seems to be happening quite a bit recently with different clients and no simple solution.