If Greenwood knows what is best for his career, he should not even consider going back to United. There are too many bad vibes in the team, and there will be too much pressure on him. If Marseille won't be able to keep him, then he should look for another club, preferably a club outside of England, where he can grow.
He's a very good player and can play in any of the clubs in Europe, but I think he should remain in Marseille. They've offered him a home when he was almost lost, and they trusted him and believed in him. Except they can't keep him, he shouldn't be thinking about leaving.