Every player, in at least the spanish BBVA league (don't know how it is with the lower leagues), has to have a release clause in their contract. Usually on these "star players" the release clause is ridiculously high (like 300mp or so), so a better question would be.. Why would Real Madrid have such a low release clause with a player of Varane's calibre? Personally I doubt it's that low. But on the other hand, without any more research than a google search, it looks like his contract was signed 2011, which is way before he was worth his current value, so it might just be.