How to add Discount Amount not invoiced in Saved Search?

We are creating a report of merchandise backlog and associated dollar value on a sales order.

To do this we are calculating dollar amount as follows:

sum of (quantity-quantitybilled)*amount

We also need to take into account discount. And so where the sales order has no fulfilled items, we subtract the sum of all discounts.

When a sales order is partially fulfilled and invoice has been generated, the discount is amortized. In this case how do we determine the discount amount that is pending invoice?

Thanks much for your help!