Even with the receipt? So you think it's possible that she undid the sale? If so, hopefully there will be a transaction where it was reversed.
I don't know what she might have done. My brain doesn't think that way and I've never worked retail, so I couldn't even make a stab at it. Sorry. I just feel certain she has her bases covered, somehow.
Does he go back and fill the card up at a teller's booth inside Wal-Mart or do they just do them at the checkout? Does he go to the same Wal-Mart every time or maybe most of the time?
Any chance he has repeatedly seen the same employee when he does that, like they might know or recognize him?