Starting from version 18.104.22.168, you can easily give a discount for some or all products/persons in a bill.
In the payment screen, you have a new button to give manual discount.
You can then select the discount (these can be made in Settings => Products => Discounts) and the products to assign them to, by default all products are selected. Per discount you can configure which products are allowed in the ‘product limit’.
It will also show you the original price, new price and total discount given.
There is also a report that will show an overview of which discounts were given: https://devblog.bmileisure.com/2020/01/16/reason-of-discount/