I would go over to his house, hopefully his mom will answer the door. You be like, "oh hey man, the other day, I was looking for some herb, not tobacco". Ask him nicely for the herb, if he refuses, whatever. Now his mommy knows that her son sells dope, hopefully knocking some sense into the sorry bastard.

It's only $20. No need to start a war over it. Pass it around that he ripped you off, never buy from him again, and hopefully no one buys from him again.