They way I understand it, and this is all speculation I don't know if that's true, is that the deferred payments (€16 million) are included in his contract with Barcelona, and Barcelona are trying to get out of that if he goes to Manchester United. There also seems to be some issue with his agent's commission, but I don't know enough about that.
As for De Jong, I was just clarifying his situation, he doesn't really need to do anything because he has all the leverage. The only issues with that would be if Barcelona default sometime in the future, particularly if they continue as they have, and if he gets punished by not being played (this is usually illegal, but it's possible).
To be honest if I were him I'd just stay at Barcelona and hedge my bets on them not going tits up.