IMEI doesn't match the box

I bought an iPhone (64 GB sim lock free) from an Apple Store in US, seems like original package, and came back home to see that the IMEI no on the box didn't match the phone and the phone was not working. Nobody touched the phone so no way to switch the phone.

I called Apple Care and they told me that the phone I got was a phone registered with Horizon (sim locked, 16 GB), but not registered to any name and I was not supposed to have it, and they told me to send the phone back to the store, which agreed to refund. But I think at the same time Apple Care registered the phone to my name. The store refused to accept the phone and when I called Apple Care again they told me that since it was registered on my name they could not do anything about it.

Any ideas how to proceed?