I have encountered a scenario here at my client.
My client wants that the billing document should be created with reference to Sales Order but only that value of invoice should be relseased to accounting which is delivered.
Sales Order = 10 units
Billing Document= 10 units
Delivery = 5 Units
Released to Accounting Value = Eqvalent to 5 Units which are delivered.
Reason why Sales Order and Billing should be of same quantity is that when a customer asks for an order confirmation, he requires Billing document for legal purposes but they want to hit their G/L's only on the basis of Quantity Delivered. As they are not aware when the customer come back to pick his remaining quantity. So they do not want to hit their accounting based on Sales Order quantity.