| murph pax |
What you all need is a tiny book (almost a pamphlet) by Frog God Games called "Dirty Tricks". It is directed at the GM and specifically covers that situation when your party gets too rich too quickly. DO NOT take the treasure away. Convince them to give it away in exchange for influence and/or renown.
A few ideas that I remember are (a) the tax man cometh, (b) please save us, rich guys, and (c) you are just the guys for the job.
Maybe someone with detect magic notices that mega-dagger or someone hears of these treasure hunters who are kicking butts and taking names. He/She alerts the local lord who either sends the taxman (they must have cash too and he wants some of it), or summons the party to a meeting. Maybe he makes a deal that they can provide service in lieu of cash. If you don't want to involve the local lord, it could be a local church and that dagger is one of their relics that was lost. Of course they will offer a finder's fee. If the party is not greedy, this encounter could establish a nice contact with some clout in town (or the whole region if your party travels a lot).
This next idea does not aim to take the dagger from the monk, but makes him/her a target. You could pull a Seven Samurai game - the ultra-poor villagers request aid. They are hoping for an army, but they can only afford the party of neophytes. Plot twist - while the bandits are attacking, one of the lieutenants recognizes the relic "Pinky of Edward Scissorhands". Every bandit turns and chases the monk. Chase scene ensues. Even better if this happens at 3rd level when the monk can barely outrun them (and feels grateful for that +10 speed). The monk saves the village by becoming the bandit king's target. Thereafter, the party has a "disagreeable relationship" with a villain. Various assassins/bounty hunters/thieves join in (a la Baldurs Gate). And that bandit king has business dealings with the bigger organization that you eventually want the party to tangle with...