I am guessing QR code remains same which points to the account of the petrol pump owner. They must be getting confirmation of payment through text or device Paytm must have provided them with. This system is not going to work long term with custom devices as they are likely to go bad. Also relying on internet connectivity in this case would be stupid because it also tends to go down. You would not want a situation where the customer has paid and you have no way to confirm that payment.