What right have Manchester United got to stop Gabriel Heinze moving to Liverpool?

Ferguson is quite happy to take Tevez from West Ham but will not allow his player to move to another Premiership team. Heinze wants to go and that is his choice as Man Utd no longer want him. They are also backtracking on a letter given to Heinze stating that he could go, now saying that it meant to a foreign side. What a joke. I think that Fergie has got carried away with his Knighthood and believes himself able to overrule the wishes of the individual.