Obviously, first thought is they are cashing in.
But as he is out of contract in the summer I can't imagine another club paying for him when he'll be free then.
There may be some compensation due because of his age, but I don't really know how that works tbh
It's something like we're due compo if you offer a player a contract at the same terms or better and he's under 24. The player can refuse to accept but the club will get some dosh. A high profile case of this is Danny Ings to Liverpool although the Fa have flaffled about with that.
Sent from my MotoG3