Hi Amlan,
I don't think this requirement is unique. The standard way to solve this would be letting the requestor to issue a material reservation rather than a purchase req. Such reservations can be created automatically from PP/PM/PS, etc. or created manually using trs MB21.
If there's enough stock, the storekeeper will issue the material for this reservation. Otherwise the normal MRP run will create a PReq. Once the material has been purchased, the storekeeper will be able to fulfill the reservation.
BR
Raf