Clause does make A LOT OF SENSE! Lets say he is a ~ 50M striker, has his prime years ahead of him and his next contract would be at around 10mln(b.t.) for the next 5 years.
If Juventus were willing to settle for basic value(crazy idea) this would have costed RM>100M without this clause. Add to that should we sell him to another team they get the lion share of the profits.
Short answer: they have imho a rebate of ~ 20mils over his realistic value. They can always use those money and add them towards his contract... He is unlikely to be a starter, but he is homegrown player for them and could be the spanish poster boy in the euro...