What they're saying is we have to buy him off West Ham to do the deal as as far as they're concerned West Ham own him - however, far as he and Kia Joorbachan are concerned they still have the rights to the player, as only West Ham cancelled the contract, and it takes both parties to do that to nullify a contract, and thus United have to deal with Kia Joorbachan and purchase him from them not West Ham.
So basically, both West Ham and KJ claim ownership of the player - clearly Utd seem to think its KJ they have to deal with. However, the fact KJ seems to be still claiming ownership seems to fly in the face of the Premier League enquiry's findings - so if the deal is done despite West Ham being against it then it will kind of proove Shef Utd's case - which is prob why the Premier League are poking their nose in to try and stop the deal!