Discount in Sale Invoice, Discount by Weight, 1 Rupee Per Kg/Liter

I want to add the field of trade offer of 1 rupee per kg/liter for Ghee/Oil Distribution Setup.
Example. Dalda Cooking Oil 5 liter Tin (Rs. 1000). Apply Trade Offer of 1 Rupee, Total weight 5Liter (5 x 1) 5 Rupee Discount Applied… Kindly Help