MRP-we have negative quantities on bills of material to recognize reprocessed raw materials. Is the

Is there a way to capture negative quantities in supply/demand or MRP messages?  We are using process manufacturing with co/by products however, we are generating a material that will be returned to inventory as a raw material in the bill of materials.  Supply/Demand and MRP messages do not capture these quantities.  Is there a way to include these in the planning process?  We want to procure only the portion of the raw materials requirements not covered by this return to inventory.

Many thanks for your assistance.