You really seem to not get what a buyout clause is. If he has a buyout clause we won't pay more than £25m, that's literally the function of the clause, it means a club has to accept that much money for the player.
The player-City negotiations are completely separate and our usually pay structure would probably be more than enough to convince him to choose us over any other clubs, not to mention the Pep effect and near-guaranteed CL football.
And we all know what would happen then - VVD would turn to shit and Denayer would become the best defender in the country. I'm not giving up hope on Denayer, he's still got 3 1/2 years left on his contract and he's only 21.