回复：今天在WalMart碰到一件怪异的事, 怎么办?

Two naive points:
那美国人也没给我要任何报酬
我还觉得应该带一个小礼品去

I would suggest to contact cell phone company to watch for the calls from the phone, ask wal mart safe guards to witness the handing over.
Don't fall in any trap. Don't go to his home because his home is near wal mart, etc.

I once picked up a cell phone in wal mart parking lot. I simply handed it to wal mart customer service. They are supposed more professional to handle such things.