PO Invoice Approval

Befiti

My client needs a workflow which enforce an approval every time an invoice is posted from the PO.

Example: A quantity of 2 is order on the PO. Upon delivery of a quantity 1 of the item, he receives and invoice the first item arrived using the qty to receive and to invoice on the PO line. He wants to have an approval. So fat so good. Now, he receives the second item and wants the second invoice to be approved. But, since the PO is already approved, no approval request is require. Is there any way to setup the Workflow to make it happen? This should apply also for the separate PO lines.