As a very rare thing, that's ok to do once as an adventure hook, sure. But in general, as Andy Slack said: if the player has invested in building up an ability that high then she's saying that she wants her character to be good at it. And that's fine.
There are carrots I can use as money sinks that work perfectly well to get rid of excess liquidity but that also have the party feel that they are advancing and the player gets to feel that she's great at something. There's no need to consistently punish them for a perfectly legal, rational choice. It's not as if the party having a lot of money is game-breaking for a PoD campaign, anyway: it's supposed to happen.
That said, if I run a "normal" campaign I'm adjusting the decrements and increments on the 3D6 broker table from 5% to 2.5%!