Hi,
AS Jurgen already explained, "it does not reflect the reality", the requirement is not logical requirement. If you still not able to convince the customer, go for development using the BAdI: ME_PROCESS_PO_CUST. Write logic in such a way that message will be issued if there is a PO in EKPO for the material for which open quantity (EKPO-MENGE 
EKBE-MENGE ) is greater than zero.
Regards,
AKPT