What the constitution has to say about will affect the formalities but at the end of the day this is a political question rather than a legal one.
If the USA wants to take Greenland, Denmark and Europe won’t actually be able to stop them which means a political settlement that gives it away.
But it isn’t a thing. These valuations are not based on reality anyway. It’s not about cash, it’s about power. Cash is fungible with power but it’s not exactly synonymous.