it's simple man, at the start of last season West Ham did a dodgy deal with a dodgy guy called Kia Joorabchian for two Argentine players, Mascherano and Tevez, basicaly Joorabchian owns the players, West Ham just get to benefit from their skills for a season, this move however is illegal...

West Ham then s**t their pants, so Joorabchian sells Mascherano to Liverpool with minimum hassle, then Tevez plays a pivotal part in West Ham surviving relegation, basicaly saving them from certain doom, so Sheffield Utd who were relegated kick up a fuss about the Tevez deal, saying it was illegal and West Ham should be punished, but West Ham say 'no no, he is our player it's all above board'...

then United come along wanting to do the same kind of deal with Joorabchian as Liverpool did for Mascherano, but they can't because West Ham have since claimed that they own Tevez to avoid being penalised for the dodgy move in the first place...