I wouldnt think that we are entitled to insert any sell on clause as he is no longer our player so any contract between him and another club is purely that nothing to do with us. I may well be wrong but that is my opinion.
BG ; Half these rules and regulations in place in the premier league and football leagues around the world would never stand up in court anywhere in the world. The clubs abide by them or they would be thrown out of the League.
If some individual footballer stood up and challenged a lot of them then they would probably win hands down in the courts, That's virtually what Jean-Marc Bosman did which changed football employment laws forever