I'd sell stones for 40. We signed him for 3 million and to profit 37 million is good business sense if the majority of the transfer went on investing in the team.
personally I want stones for the next 10 years at the club, think he is a fantastic player. but all this he will be worth more next year talk all summer, that's not true at all. How many defenders that young go for that amount of money? We have turned down 30 as we want more fair enough, but us asking for at least 40 doesn't mean he will go for more next year if we didn't sell.
and if Chelsea are that he'll bent on signing him then why would you keep him for a year if his value doesn't increase? may as well get his replacement now and invest some money into the team. even if that meant the money was transfer dependant all along, signing a good replacement and making sure we got players at the level of yarmal would make a much bigger difference to the first 11.
We can replace stones, but to improve the team we can use the cash from the sale to do that.