We should demand our money back.
Sure the idea sounds facetious but there's a method to my madness. Forget for a moment that government money vanishes like a fart in the wind once spent. And also forget that government spending is a one way street - no entity ever returns taxpayer dollars saying, "no thanks, we've got plenty already". You and I know these are the cold hard facts of reality but who says arguments have to be based in reality these days?
Whether or not the money is ever returned is not the underlying point. Just the act of making the demand will send out its own shock waves and also serves several important purposes as well. First, it plants the seed of the idea that the $ 500 million was illegitimate in the first place. And such and idea brings with it the implied connotation that the money was stolen from the hard working American taxpayer - and by God we want it back. For good measure we'll have a few politicians get in front of a microphone and put on their angry faces shouting, "I'm outraged" and "save the children". That's always a nice touch.
Secondly, it forces the United Nations to respond. It doesn't matter that they'll offer up a canned response of some sort of babble or another, what does matters is that they must respond. Remember this is an entity that enjoys operating in the shadows and when you flip on the light they scatter like cockroaches. The last thing in the world they want is attention of any kind.
And once they do respond you set the hook. The device is known as the MacGuffin to fans of Hitchcock and its only purpose is to create an air of suspense and mystery. Examples are on display each an every day, especially these days where seemingly tough, interrogating Senators intensely "investigate" Trump cabinet appointees. "Are you saying Mr. Jones that this million dollar line item expenditure is not right here on this piece of paper I'm looking at right now?" It's just a ruse that makes for a dandy sound bite for those viewing at home.
Same thing with the $ 500 million payment to the United Nations. Demand our money back, wait for them to respond, then set the hook.