This is just one of them badly thought out rules.
Common sense suggests that if Nice accept an offer of lets say, 40m from West Ham, in what world should United not be able to match that.
Even if no other clubs were in for him, if fair market value was reached, which I know is always opinion based, then you shouldn't be able to stop a player choosing his preferred place of work.
Again, common sense should be used in situations like this.....Whereas, you have Kellyman moving to Chelsea for 19m, which is clearly way above market value, effectively committing a dubious transaction to cook some books, common sense should also determine a block on said deal