We withhold retainage on Construction AP Invoices. On AP invoices, we often can take a discount. W

We withhold retainage on Construction AP Invoices.  On AP invoices, we often can take a discount.  We are currently setup to Prorate the discount and it prorates across retainage as well. The users do not like it reducing the retainage,  saying that their suppliers still want the full amount of retainage paid.  Is there a way to exclude retainage from the discount?