Not saying it's going to happen, but Stones is under a long term contract, Martinez doesn't have to accept his transfer request. Look at WBA they rejected Berahino's and are still rejecting bids from Spurs. Also, if we did agree to sell him, why would Chelsea bid much more than what they have already, now knowing he wants to leave gives them better leverage. If he was sold it would be surprising if we got anywhere near the £40m we all assume he's worth, a lot of any deal will be performance related.
Stones had a very steep learning curve when he became a regular last season, what's to say Browning can't do the same.