Now this is a funny story!
I admire your tenacity of purpose to revenge yourselves for the 49 bucks he cheated you for. 50$ for a donkey is a bargain, we charged 100$ as shopping fee in my towns back when the money was needed for building. And God, both Zeus and the Western God, knows that building towns is hard to do early in the game (The West). Not that anybody have cheated us, most people donated more than the required 100 bucks, they wanted to be in good standing so they could get an another shopping invitation later on. Now a days most of us can earn in a couple of days what it costs to challenge a big fort.
Myself, I have always donated a nice sum as a thank you even in towns we have shopping agreements with. That leaves a nice impression.