Is it really an unwritten rule that you don't quit winner?? Simple answer, yes! Unfortunately, that is where most people stop, and then they end up distorting WHY that is the rule.
It is NOT a rule to make anything fair. That is just loser mentality. The reason it is a rule is because the whole idea is to make money. If you quit winner, how much money did you just throw away?? I say that, because of how much more would he have lost that you now won't get?? Quitting winner USUALLY, but not ALWAYS means that you just robbed yourself.
Now, if the game is much closer than you liked, and you feel you just played your best, and suspect that he is just warming up, then you are a loser if you DON'T quit winner!
Guys, the whole purpose of gambling is to make some money! You have to know when to hold them, and when to fold them.
Now, if you are trying to string some guy along, and he gets the first set or two and then quits, well, too darn bad. You took a gamble on what he would do, and you were wrong! Own it and face it. It happens. That's why it's called gambling. Some people you can string along, some are smarter than that and quit when they should. It's all part of the "game".