Hi SD Gurus

My customer kept an order for a value of Rs10,000 for material M1, price of M1 is Rs100. Due to some non availability issues i want to do material determination with M2, price of M2 is Rs200 which makes the order value Rs20,000, which shld not happened. where shld i have to customize so that the qty of mat M2 shld update as per the order value which is Rs10,000

Plz help me out