Invoice Qty should not exceed from Sales Order Qty

Hello everyone,

     I have came across a situation that say I have created Sales Order for an item 1000 qty. Then I created Invoice partially say for 400 then i create another invoice say 300 and then I create invoice say 400 again,then It should notify me that the qty in Invoice exceeds from Qty in SO.

I have done some SP, but it works only if I create one sale order for 1000 qty and then if create on invoice of more than 1000 qty it notify me. But I want it for the case described above...

Anyone who has SP written. Plesae share

Thanks in advance