Would it be crazy to pull up to said person, say, "I'll give you 50$ for that bike, right here and now." and expect to get it? Maybe you should throw a slightly larger bike into your truck so when you see him again, you can say, "Hey, I had a bike just like that when I was a kid, I'll give you 20$ plus this other bike that will fit you better if you'll let me take that one." That way you won't let on the fact that its valuable.

Good luck!

peace,
sam