Dear Sameer
The suggestion told by Mr Lakshmipathi has already been implemented in our organization.
But we are doing the same check at contracts through VA41.
Our client have bookings for some materials before the creation of contract.We first collect the amounts from customers as booking amounts with the custom Tcodes(this Tcode will update both material +Qty+amount+Season into a custom table-also this Custom Tcode will run the Standard Tcode F-21 and stores this accounting doc at special GLs assigned-later while creating contracts we made the same checks in Sales exits level MV45AFZZ)
you may suggest the same process to your client where-they can save the advance payment either 50% or 100% into a custom table-later having the checks at delivery exits level as suggested)
Hope this helps you.
Phanikumar