According to the Guardian, it seems that Man Utd. tried the same trick that West Ham did with Carlos Tevez trying to prevent Tim Howard playing against them after he formally transferred to Everton at the beginning of the year. But then Utd. agreed to remove the clause from the contract provided they got "a gentlemen's agreement" that Howard would not in fact play against them. In the event, in one of the most crucial matches of the season, Eveton did not play Howard. ManU. were 2-0 down well iinto the second half and looking down and out when the substitute goalkeeper gifted them a goal and basically gave them the hope that got them back into the game, which they went on to win 2-4, as Chelsea were dropping a couple of points at the same time.
So when Mourinho and Wenger say it is one rule for Man Utd. and one rule for the rest tehy do have a point.