Without US money the UN would quickly devolve to the condition of the EU.
Real power brokers, what?
True, but the EU still wields considerable influence, and so would the UN at least for a time. The US must not relinquish its leverage in that body until it is so withered that it can no longer cause any real trouble. The best solution would be for the US to choke off funding to the point that other countries start bailing out first. Once the exodus becomes significant, it becomes much easier to walk away citing a general lack of participation.
Not to mention, if we stay in for a while after we cut down the influx of cash, it’s easier to deflect accusations of deliberate sabotage. After all, “we’re still participating, right? Surely the UN is about international relations, not money”?