I believe what Trump said is just a political stunt,he did that to serve as an excuse not to travel to Norway because Obama is also visiting at that period which h might affect his upcoming election.politics
Green land is very important to the us because the United State military has some important equipment there,
Russia and the United State also have military equipment in green land,so if the United State buys it it will be there own and Russia could be expel
Donald Trump's is a real estate manager he may have some interest in aquiring green land to build some houses
Green land have a lot of mineral resources,which can be of benefit to the United state