Automatic downpayment posting during sales order

HI All

My client requirement is to automatically post downpayment at the time of sales order ( VA01). After delivering and billing the same downpayment need to be cleared.

Our Sales Order requirement with an eg

Material=X , qty=1, Amount = 1000

Here we want to trigger Rs1000 as automatic downpayment as and when we save the sales order.

Anish Shaji