I'd go there and playfully mention, "What ya going to give me for it?"....if they offer something, then that's cool, if not, I'd probably give the phone back anyway.
Someone screwed up. You guys never screw up something at work? Don't be a douche-bag and demand a $1000 gift card or anything. You could very well be affecting someone's job right before Christmas. Their mistake or not, you took advantage of a situation. It's like blaming some old lady you scammed out of her retirement money because she "should have known better." You had a fun couple days with it, just give it back. You can wait a few more days. They may even hook you up.
WITH THAT SAID....
If the person at the store is an ass about it.....go ahead and keep it.