When Leicester get relegated (and they inevitably will) I reckon a £20m-odd offer for Mahrez would force their hand to sell.
He'll hand in a transfer request and they'll want to get rid of his big contract and raise money for players who will be mentally there to fight for promotion. Also if they hold on to him, they risk him devaluating even further after a year in the Championship. Nobody will bid £30m+ for him, because they'll know Leicester need to sell.
At that cost you have virtually no risk. If he doesn't kick on after a year or two, he'd still be only 27 so you sell him for £10m-odd and recoup half of the cost. But if he kicks on.... There's still a good player in there, I can see it.