service purchase order with network account assignment

purchase requsition made with N acct ass and D item category, value 1000 $ but quantity in PR given 1 now purchase order made from this requisition of value 700$ and quantity 1, migo miro also done......System is not allowing to use remaining 300 $ in PO saying quantity fully used. how to use this remaining 300 $ . in services tab service no given which is made in project system..pls help.. also purchase req value cant be changed to 700$