Change quantity of a Purchase Requisition from external operation.

Hi,

how can i change quantity of a Purchase Requisition generated by an external operation inside a production order (creation indicator F) ?

The problem raised when the production order has quantity 10, (reservation of purchase requisition is immediate) the external operation create a purchase requistion quantity 10.

But at the previous operation of external operation i scrap 3 pieces, so i need to modify quantitiy of purchase requisition, becouse i have to issue only 7 pieces for the external processing

thanks