Purchase Requisition - Unordered Quantity

Hi All

We're on R12.2.6, we created a purchase requisition with line quantity 100, then we used auto-create function to create a new standard purchase order with 60 quantity and get it approved. later on, busineswe abandoned the reaming quantity i.e. 40. requested in purchase requisition

How can we Cancel the 40 quantity remaining in the purchase requisition? as it still incurring the budget !

Thank you in Advance