Lots of internet tough guys here..Bottom line is that if someone stiffs you, your options are limited. You are seriously going to beat a guy up over 100 dollars, 200 dollars etc..?Then the guy runs to the police and you go to jail. Basically your life is ruined. Even with such a large sum, that's nothing compared to your freedom. Would you spend 10 years in jail for 90 000? In that case, you are an idiot or have no future prospects. If you get convicted there is zero chance you get to keep that money anyway.
Then there are those who have watched to many mob movies and wants to kill the guy...Even though it would be perfectly obvious who the prime suspect would be. You are not the mob, you don't have judges in your pocket or huge resources and people to cover your tracks...But that sort of thing does happen in every country. I was once introduced to, and had a long conversation at a party with a guy that later screwed some yugo gangsters out of about 150k. Didn't have a good feeling about that guy, he seemed very "slick", but I had no idea he was a criminal. He disappeared. I don't know what those guys do to people exactly, but I can't imagine his last hours being very pleasant.
Bottom line: As a "civilian" your options are limited when a guy stiffs you, unless you are willing to risk everything to get the money. And don't screw criminals over, because they are willing and able to make you disappear.