AITA for telling my BIL to pay me $700 for my lost coat when I bought it for $100?

I was at a shopping mall with my fiance last weekend when a young guy tried to sell me a very nice, full-length camel hair coat for $100. I liked it and bought it without hesitation. When my fiance saw me wearing it later, he told me that I looked “very wealthy” in it. I took that as a good sign and have been wearing it every day since.<br><br>A few days ago, while we were visiting my fiance’s family, I told them how much I liked the coat and that I just happened to buy it for a very low price from a guy in the mall. His sister said, “Oh, you mean you bought it from that guy Jamie? I think he bought it from a guy in Los Angeles for $700.”<br><br>I was shocked, to say the least. I admitted that I didn’t know Jamie was the owner of the coat and that I had no idea he might have bought it for an an order of magnitude higher price than what I paid. My future BIL said, “Well, actually, I’m the owner now, so I guess I should get a refund of $600 from you.” I argued that I had no way of verifying whether he owned the coat or not, and that his $600 demand was absurd. He said, “Well, I’m calling you an untrustworthy cheat for trying to steal his coat for a fire sale price.”<br><br>I upped my offer of compensation to $700 because I honestly don’t plan to give him a dime. He is calling me an asshole for refusing to pay Jamie a large fraction of the coat’s supposed retail price, but I think the guy in the mall should have gotten fair market value for his coat, either from me at $100 or from Jamie at $700.<br><br>Am I an asshole for not giving his coat’s “actual” owner a large windfall of money on the grounds of guilt by association?