Accounting for cashback received from paytm, freecharge etc.

This issue was faced by me in the audit. I will take an example to explain the question.

Elec. bill payment of rs. 1000 was made through paytm. Please note that this payment is not like first money is loaded in wallet and then payment is made but it was directly made from the bank account through paytm. I received 100 as cashback.

Then another mobile bill payment of rs 500 was made for which 100 cashback was used. So payment from bank was rs. 400 only.

Please tell me the accounting of both the transactions and also how it will settle in end because ultimately everything is to be trf to p&l or balance sheet.

Thanks to those who will try to give the answer.