If you purchase scrip at a rate that is different than the default discount percentage shown when entering inventory, you can simply change this percentage right on the 'Receive Inventory' screen.  This change effects only that purchase, and does not change the default discount percentage.

When you enter a sale, if there is no inventory shown for that vendor, the sale gets calculated at the default rate.  If you then receive the inventory at a different rate, the software knows which sales are effected by that inventory received and will will re-calculate the sale at the correct percentage.  This does not affect previous sales that have already been filled.

Because of this method, you DO NOT have to update your vendor's discount percentages before entering your sales (unless it changes permanently).  You simply have to enter the percentage on the 'Receive Inventory' screen exactly as it shows on your receipt, and everyone will get the correct rate with no adjustments needed on the sales side.