The point here is he did trust you enough to make the deal in cash. Finish the deal and move on.
Oh Lordy, talk about dumb and dumber! This guy who purchased the bike is not playing with a full deck. Anyone who would give a perfect stranger a handful of cash, no reciept and no title has just a wee bit of vacant air space between the ears. Then for the fella who took the cash to play fook fook with a character who has proven himself as unstable is also in need of a few session on the sofa with the man with a bow tie, wide rim glasses and a note pad.
If I were you, I'd put motion dectors around my house and tie up a pit bull on my front porch if you intend to keep playing with this guy "Over Principle". I'd say the wise thing would be to do whatever it takes to keep this guy out of your county! Keep in mind, the guy knows where you live. It would be a shame to get a phone call at work that your house has just burned to the ground!