Trump seems to be under the impression that sovereign purchases work as they did in the 1800s. When the United States bought the Louisiana Purchase from France, the people living in that region had no say. When the United States bought Alaska from Russia in 1867, there was no referendum among the 50,000 or so indigenous peoples living there. But this is 2019.
What’s more, the relationship between Greenland and Denmark is not as simple as that between colony and colonizer. Especially since a 2008 referendum, Greenland has a far greater measure of autonomy than earlier in its colonial history. The country has its own parliament and premier and control over most political matters, except those concerning foreign policy and national security.
Sure, Trump’s Greenland fantasy is a long way off from realization. But if it were to happen, it would be through a process that Trump has yet to invoke: one that depends on approval of the transferred territory itself, ideally through a popular referendum or legislative vote.