Except Trump has no sense of humor so it was probably serious:
At one point last year, according to a former official who heard him, he even joked in a meeting about trading Puerto Rico for Greenland — happy to rid himself of an American territory whose leadership he has feuded with repeatedly.
Nobody is really shocked about this are they?
Trump is gonna try to throw Puerto Rico into the deal next isn't he?
— Exhausted Cynic (@CynicalCBlog) August 16, 2019