He makes no sense, if you are going to sell an asset then you wait until it at its highest possible level. If you think that Stones is going to be the best English centre half in the country and the future Captain of England then you have to back your judgement and sell him in a few years. To do anything else makes no sense at all. In effect that is what Mourinho is doing now, he knows how good this lad is going to be and that's why he is trying to buy him now. If he waits a year or two then he may get out bid for him.
I hate to use them as an example but look at Suarez, Liverpool refused to sell him when Arsenal bid £45 million and sold him a year later for £75 million. Stones's price like Suarez is going to go up therefore you tell the player and Chelsea he is not for sale at any price.