Carmine, Greenland does not own itself. It is an autonomous territory of the Kingdom of Denmark whose economy is HEAVILY reliant on aid from Denmark. Therefore Greenland isn't calling the shots here on a sale. Denmark is.
I am going to point you back to the precedent of France selling Louisiana to the USA. This was a very sudden and unexpected deal. France at the time needed money badly to fund Napoleon's wartime enterprises and conquests, which costed it a lot of money. Napoleon said, "who fucking cares about Louisiana? It's a big fucking swamp!!!!" When the U.S. offered $15 million and it was accepted, Monroe and the U.S. administration were delighted. Nobody in Louisiana at that time saw it coming or controlled the deal.
I could envision a scenario where Denmark gets offered money, and also gets offered to be relieved of its burden of providing aid, and says good riddance to Greenland. Much like Napoleon did to Louisiana. You seem to either discount, or be totally ignorant of the possibility of the U.S. making this happen. I am here to tell you that the U.S. can make this happen if they want to.