If Real Madrid brings him to the team, I think it would be a big deal because Real Madrid still has problems in the center back area. Due to these problems, they can concede surprise goals and get some unexpected results in matches. If we think about Leverkusen, it will be a big loss, but I'm sure Xabi has thought about who to get instead of him. If Tah leaves the team, there will be problems in the center back area.
So far, Xabi Alonzo is going to be a good manager at this time. I don't really think he gave it away his potential player to Real Madrid. Like a post above, Jonathan Tah is effective role in his team, if Xabi sold it, he needs a same potential player like him, and it's difficult right now, because all potential player for his position has been recruited by all the best club. Xabi must start from the beginning again, and search for the young player who has not been formed. So, he must be thinking it again, at least if he really want to sell Jonathan Tah, the price should be higher from the offer, so he can find a good replacement and get additional player for another position.