Why would they? AFAIK, Denmark doesn't have much debt, they have a stable society with good healthcare, low crime rates, etc. If I remember correctly, Greenland has a bunch of rare earth metals which could be worth a lot if they would actually start mining up there. That said, in all of this mess, a good thing coming out of it would be that Trump fired up NATO enough to deploy more troops and increase military presence up there (if it's so urgent as orange man wants it to be). Why are you so certain that DK will let him buy it?