回複：今天在WalMart碰到一件怪異的事, 怎麽辦?

Two naive points:
那美國人也沒給我要任何報酬
我還覺得應該帶一個小禮品去

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.