> acquire land via direct negotiations with Greenland natives

"Greenland does not want to be controlled by the US. Greenland does not want to be owned by the US." - Jens-Fredrik Nielsen, Greenland PM

This view is shared by ~95% of Greenlanders.

You can't unilaterally enter "direct negotiations" with people to purchase their belongings. The active and willing participation of both parties is a key element of a genuine negotiation.