I have quite a few thoughts on this, so bear with me as I may ramble a bit. For everyone saying the OP did nothing wrong, in terms of nothing legally wrong you are all correct. But based on the discussion going on in the other Best Buy thread, the OP and the other AC member who purchased phones had a pretty good idea ahead of time that Best Buy was not supposed to sell the Nexus on 12/11. They and numerous others actively contacted Best Buys hoping to find a store or employee who was not made aware of the delay. Everyone can read the other thread and see posts by members either calling or driving to numerous BB stores trying to find someone unaware of the delay in release. In addition to that, a few members who are BB employees identified the 12/11 date from the screenshots as an in stock date, not a date on which the phone could be sold. Was it the fault of someone at Best Buy that all Best Buy employees were uninformed and not properly notified the Nexus should not be sold? Absolutely. But just because you CAN take advantage of the uninformed, doesn't mean you SHOULD.
I have not seen the OP say the BB employee is demanding the phone back. BB is admitting they made a mistake and is asking him to return it. I have not seen anything from the OP indicating BB is threatening any action, they are simply asking him to do the morally right thing. It is obviously important to them, hence the numerous phone calls. This is a case of two wrongs don't make a right. True, he has no obligation to take the phone back, but most people would probably agree that returning the phone is the right thing to do. Are common courtesy and common decency really that dead in this me, me, me world?