very important and urgent

suppose I have an agency of some selling company
1. Firstly they sold me products for eg at rate of RS.1000 including GST and I have to make them payment at the time of purchase
2. As per company instructions I have to offer discounts to customers for eg at the rate of 20%
3. I cannot charge more than at the purchase price but as I also have to given 20% discount the amount I received is RS.800
3. Company gives me commission on sale as well as I raised debit note for discount offered in consolidate

now what will be the treatment in 3b and 1
more over how should I raise invoice
because sometimes customer buys multiple products and I may involve numerous calculations because percent of discount directed by company changes from time
and it becomes difficult to raise invoice