Personally I think the same person coming back in is a red herring. Doesn't matter who comes back in to buy $70 dollars worth of his useless made in China stuff (hahaha) and get $30 back in change. Shop owner made no profit.
He's out $100... $70 worth of groceries plus the $30 change he gave her.
Maybe it would be easier for people if they reverse the order. A lady comes into a store and buys $70 worth of shit. On her way out, she steals $100.