Proforma Invoice

Hi Gurus,

I have requirement in which Proforma Invoice is to be created on the basis of Sales Order.

If the sales order has line item having 100 Qty.Now if proforma invoice is created the line item will have 100 Qty.

Now the client wants to add 120 Qty in proforma invoice which is not allowed.Any Qty below 100 is allowed but more than 100

is not allowed.For that i informed a workaround as to change the sales order qty and create an invoice and again revert the changes in Sales Order.Is there any other way i can do in respect to configuration.