He should have left this summer but he chose not to.
That is on him and you can just imagine the response he is going to get from the fans when he plays a game.
This perspective is over the top.
When clubs want to buy players, they go to great lengths to convince the player, the agent, the selling club, and a host of other concerned parties. A contract is then prepared and that contract is honoured. Just because a player doesn't live up to the club's expectations, the club cannot show him the door unless it is mutually agreeable.
The fans can boo Maguire for playing trash football and costing United points on the field, but certainly not because he refused to join a lesser club without a proper payoff when he has a better contact at United. He doesn't have many football years left in him either.