Tell him to fuck off. He's not going to stop being friends with you because you didn't pay him ten bucks. If he does, then you really should have never considered him your best friend.
A week or two after it blows over ask him what the heck it is that got that into his head. Let him know it wasn't ok. It cheapened your friendship.
I've learned that some people just do this without realizing that it's wrong. Something about their childhood or whatever. Telling them it's wrong won't stop them from doing it completely, they don't think it's wrong, but it will give you further justification for refusing to bow to such nonsense in the future.