message when changing order qty < confirmed qty in PO

Hi

How to handle the following scenarion

We place a purchase order for 100 Qty and the vendor confirms for 100 and we update the confirmation quantity.

then our requirement goes down and we agree with the vendor to reduce the PO from 100 to 70 and then change the PO quanity but we did not update the Confirm quantity, MRP still thinks 100 qty as the Confirmed.

Is there anySAP can issue warning message or error in PO when we change the order quantity which makes it less than the confirmed quantity

Thanks.