After reading, I have to say, rookie mistake. Plain and simple. If your going to do a deal with $400 worth of product, you wait till the shit's done and your back home to toke up. His friend definitely set him up. But seriously, your going to let him lead you to a dark ally, with that in your pocket?!! Rookie mistake.
Granted there's always a chance to get jacked. Someone thats built up credit with you turns bad. But if they do that when they've already been doing regular business with you, their dumber then dogshit. Because your never going to do business with them again, and that $400 dollars they made that night compared to the money they lost from doing regular business over time doesn't match up. Sadly alot of kids don't get this. I got jacked, for $20 dollars. Seriously, he held me up for $20 at gunpoint. Whats more, he was proud of himself. To short sided. It was my first and only time dealing with this individual, and I was only getting $20 in case shit like that happened. I really wanted an oz.
To the original OP, I would gather up my boys, but I would go and jump your 'friend'. The rest were strangers, they may be trash, but they didn't betray you. He did. That sort of shit can't go unpunished. I'd stop his face into the concrete. Be careful, you don't want to kill him, but you want it to be permanent damage to.