Hmm... interesting. This is how I would play it where I running a game with the Monk
1. At the end of each adventure the Monk must give up all of their coin, it goes to charity or just drops it on the ground.
2. If the Monk opts not to, for whatever reason, they lose their Wisdom bonus in the following adventure.
OR you can just not give the player any choice and all of the money disappears just prior to the end of each adventure, POOF! gone. Sort of like having excess Prizes, but for monies.
Either way, that is kind of interesting, hadn't noticed until now.