Upgrade gone wrong

Called the store yesterday to enquire about stock, the guy tells me rather come tomorrow as the price will be cheaper, he will put the phone aside.. Upgraded contract this morning, and i never even touched the phone, when i opened the box realised it was the smaller model, went back to the store in the afternoon to tell them it is the wrong phone, not the right model, thought i was getting a good deal for the phone i wanted.. they refuse to cancel that deal and give me the other phone on a more expensive contract.. makes no sense.. they will make more money from me, its the same day, the device is not used at all..

I use the phone for business and the phone wont work for what i need..

You guys need to help me with this