% Scheme in Billing Document

I have following scenario :

  1. In Sales Order, there is only one line item having Quantity 3000.

  2. % Scheme discount is allocated in Sales Order correctly from scales defined for 3000 Qty.

  3. While doing Delivery and PGI, the batch splits automatically splitting 3000 qty in to 3 batches.

  4. While doing billing, there are three line items of different Quantities totalling to 3000. But in billing

      the % scheme is being calculated depending upon Quantities of each line item.

      I want the same % scheme should be applied to all line items which is allocated in sales order of 3000 qty.

Can anybody help in this ?