Poor service

I went in to buy burger meal that was on special till 11am the cashier assisted me but I realised a minute later that it's not the same as the one I wanted and the transaction was already finalised.....I then went back to the cashier and asked if I could get the one I actually wanted, she called the manager and I was told they can't void the transaction and that it will take days to void. I am upset because it was literally 2 minutes when I realised the order is wrong and also as a customer I feel my rights were violated because now they expect me to have something I did not want.