Is he currently in the first team?
No.
Then they are not going to get £15m for him. At some point he'll want a move and Spurs will probably have to sell at a lower price. You can put a premium price tag on a player who is starting every week, but if he's on the bench you can't really expect to keep him indefinitely.
I'm guessing they'll be willing to let him go if they sign a few more in his position, but until then he'll be overpriced.