Hi,
Use a discount condition: XXXX
Example your contract Qty is 1000 and discount: XXXX for this entire contract is 100.
How when you create a release order which is nothing but a type of sales order for example 100 qty then respective discount will calculated as 10 as per calculation and required config at condition type and pricing level along with copy control.
Hope this will help you out.
Many Thanks
Sumeet