We usually give discount amount. Isn’t it possible to write discount amount rather than discount percentage?
If you need to enter fixed discount amount (rather than percentage), use
Add line button to create new line item for discount and enter the discount as negative amount.
Dear this method could work but it is not a proper way to discount in amount rather then figure. because what we have studied is trade discount should not be in books and if we prepare an purchase invoice we show them trade discount on it not cash discount. please allow percentage and amount for discount as well