Greenland is an autonomous territory within the Kingdom of Denmark - in 1953 it became an integral part of Denmark.
In 2009 it passed the Self-Government Act, which recognized Greenlanders as a separate people under international law and granted them the legal right to seek full independence. Greenland manages its own judicial affairs, policing, and natural resources. Denmark remains responsible for foreign affairs, defense, and monetary policy.
Besides Greenland, the other autonomous territories of Denmark includes the Faroe Islands.
Basically separate countries under the Danish crown.
“Denmark” can no more sell Greenland (or the Faroe Islands) than the UK can sell Scotland
A colony is a colony is a colony.
Legalisms do not change that reality.
Until Greenland votes for independence they are just another colony.
Its not a sale its a bribe. Just what they need to back off. And, Greenland will willingly become a US Territory like Guam or Puerto Rico.
Remember, Trump likes deals. I’m certain that he can offer the Greenlanders enough that they’d sign on.
Why Denmark thinks they should have the authority to veto such a deal, against the wishes of the Greenlanders, I don’t know.