Apple gave me the wrong iPhone, do I have to give it back?

Today I took in my iPhone 4s 16gb that I bought from a trading website a week ago. I got ripped of real bad as when I checked the IMEI for the phone it was all good, and when I received it I realised they had told me the IMEI on the sim tray, which was from a different phone, and the actual handset turned out to be blocked:/. They refused to let me return and were very rude on the phone. I thought about contacting the police but then remembered the phone was sold as faulty with 'no Wi-Fi'. I thought I might as well try my luck at the Apple store to see if they would replace it. I bought it in, and it turned out to still be under warranty! They gave me a new phone right then and there. But the thing is, the lady gave me the replacement based on the sim tray, which I knew was from a different phone, but, it turned out to be from an iPhone 4s 64gb. I discovered I now had a 64gb 4s when I got home. Should I just keep it? Will they find out?

Keep it. It could very well have been that the Apple store in question had no 16GB units in stock. If they realize their mistake & contact you, consider exchanging it, but you acted in good faith.

You scored big on that one. No I would just keep it, its not like you stole something, they exchanged and gave you that phone.

Keep it!