I don't see why the standard MM-SUS process won't suit this. Services were already delivered, thus I assume the PO will contain exactly what has been already delivered. This PO just needs to be sent to SUS as usual and then the supplier creates a confirmation that matches the services in the PO. This will create the SES in MM, which the purchaser needs to approve or reject. If approved, supplier can create the invoice in SUS, if rejected supplier cannot create the invoice. Its simple and easy.
regards
Zoltan