He clearly does not understand how international relations work today.
The borders were last changed in Europe after World War II, 1945.
As far as I remember, the last a piece of territory America bought (from Russia) was Alaska in 1867, it seems to me, more than 150 years ago.
Obviously Trump needs a history lesson.
Trump has advisers who are making things known to him. This is one of the reasons he changes his mind so often. He is learning from his advisors, to say nothing about his opponents changing the playing field all the time.
There are many borders that are changing all the time, although many haven't changed since WW2. Consider Thrace. Thrace has not existed for hundreds of years according to the historical and government books. But Thracians still cling to their heritage as Thracians, even though they are being integrated with other cultures.
Things change... even borders.
This is true, but it used to be the standard for borders to be changed by force, in war, and now it is settled peacefully, through negotiations.
200 years ago Trump would not have to ask anything, he would send army and declare that the area belong to America, but now we live by other standards and rules.