CTQ - To serve priority customers depending on quota

We wish to implement a functionality thereby serving our priority customers on quota basis..so for example, if customer X has quota of 1000 units of product 1, when he places an order of say 1500 units of product 1...the system should accept order for 1000 units but reject the extra 500 units. Similarly if he places order for 600 units first up..he still has a remaining quota of 400 units for the period, in short he can still place another order for the remaining qty.

Regards

Anurag