Cashback due to GST
This cash back is part of the GST Laws.
A simple explanation: The restaurant charges you 18% GST and pays that to the government.
However the goods and services restaurant buys raw materials to serve you food (like let's say branded atta, masalas) also has had GST paid on it.
Now the restaurant can claim the GST paid (on atta, masalas) against what it has to pay (18% on your bill) and only effectively pay the net amount.
This restaurant has decided to pay that back.
This is part of the anti-profiteering laws under GST, and sooner or later, every service provider will have to do this.
This is a kind of ethical business practice as well, because the service owner will have to take the call here to give the cash back. more