How to route approvals in Purchase order based on line discount

Summary:

My customer wants to route approvals differently if there is line discount entered for a PO line. I could not find any discount related attributes in the PO approvals.

Content (required):

The customer's current requirement is that if Buyer overrides the Agreement price on a PO line, Procurement Manager should be notified. But if there is no price override and discount is entered, the workflow considers it as Agreed Price change and still routes the approval to Purchasing Manager. Customer does not want Purchasing Manager to be notified in this case.

I could not find any variable related to discount on PO line. How to access Discount percent on PO line and how to avoid approvals going to Purchasing Manager in this case ?