So, i have an old phone, i put 100 euro on it, then put it in flightmode, and make a backup. Then i start paying stuff, and when the 100 is finished, i put the

So, i have an old phone, i put 100 euro on it, then put it in flightmode, and make a backup. Then i start paying stuff, and when the 100 is finished, i put the backup back. I know the double spend is not solved with this paper, but how does the retailer know this is a phone from a double spender? He shows a QR code to the customer, and not visa versa. The retailer does not know anything from this customer.