He will get a sign on fee from them. The only way he would sign it if he really isnt that keen on Madrid, which i doubt
IMO the smartest thing to do for his career is to make it very clear he wants to play this season, cement his place in the national team, and renew with us with a reasonable release clause that makes it pretty easy for Real or Barca to sign him next summer. Yes, he might get a signing-on fee if he leaves for nothing but he could get quite a chunk of that through salary if he renegotiates now, and it's less of a risk if he gets injured or the move falls through for some other reason. If he doesn't negotiate with us he might not play, might lose his place in the national team, lose a year of football at a key stage in his career, and also burn his bridges. It would be risking a lot just for a move to a club who have royally messed up bringing him in.
Not sure I am expecting him to do this, but I do think it is the most intelligent thing to do and his agent might well advise him accordingly.