The issue with the West Ham deal for Tevez was they only bought part of the player, the rest was owned by his agents - this arrangement did not comply with PL rules. The Rags effectively borrowed him for two years and owned him outright on loan for that limited period which was OK. When that arrangement came to an end, he was owned by Kia whatsiname's company again and we bought him outright from that same company.
The 3rd party rule did not apply to United as they did not own him, West Ham still owned his registration But Kia Still owned his image rights this is how they loaned him to United, This is why City stepped in and took him from United, because the rule was changed and you had to buy the player fully and not 3rd party ownership.
City bought Tevez fully and Kia went back to just being his agent, I also think United played dirty with West Ham and they had no choice but to loan him to United or they would have had the Premier League kick them out,
Sheffield Utd took them to court for loss of earnings and won so West Ham was guilty but stayed up because man united make the rules that everybody else has to play against