Had a similar situation, sold a bunch of parts to a guy, have a volume discount, he ended up not wanting most of the parts and asked to return, I said ok, but said that since he only bought one item no discount, let's just say he did not take well to that.
He ended up threating me on my own property, I said what ever and gave him his money back, he knew where I lived and I did not know where he lived so I figured it was worth a few bucks to have him go away.
Funnest thing is that my wife came out with a baseball bat. And after getting her to go back inside she neglected to call the cops!
All's well that doesn't end in a fight.