You can choose not to go with such person next time.
Or you can do it back to him, to get back the money. Let's say he
ask you buy once, never give you the $4, with no small change or
whatever excuse. Then within few days(dont wait too long), you give
excuse and make him go buy, and when he come back, u pretend to
take out money, but before you really hand it to him, you say "Eh,
yesterday i buy for you hor, ok, now we "bo lai" liao".