I think we are assuming too much in assuming that Leake definitely won't grant the Reds permission to send him down to the minors.
First, he gets paid the same no matter what this season. It might delay when he becomes a free agent, if the assignment is longer than 20 days, but odds are good that it wouldn't last that long, considering how often pitchers get injured. And even then, that's not money lost, it's money delayed.
Second, if he was claimed on waivers, he would likely be claimed by the Marlins. He might rather stay with a contending team in the minors than go to Miami. Playing in the postseason is why most players play the game.
He probably would not give the Reds permission to send him to the minors, but I don't think it's a given.