If City arranged a deal with Grealish and Barnett, then made an offer for the release clause and then Grealish and Barnett started discussing a new contract with Villa, City would drop the deal like a sack of bricks. We've already seen that with Fred and Jorginho and Sanchez.
If there was a release clause, it'd be over. Would you like to come here Jack? here's what we'll offer you. Agreeable? good. Here's £85m Villa. Welcome to Manchester City.
With loads of other business to be done, ingoing and outgoing, the club wouldn't piss about delaying activating a release clause.