Just go to his house and ask him. What's he's gonna do? Fight you? I doubt he'd want to incriminate himself like that? Most people would be pretty embarassed and give the money back, or run away, which then you can shake him down. Bring a couple of your boys for insurance. Don't give him verbal threats on his Nextel, fuck that shit, element of surprise. Don't be a narc when you tried to buy weed and he did you wrong. Besides it's 20 bucks. Who gives a damn.