Real Madrid have a €50 million buyback clause that allows them a 48-hour first option to negotiate a transfer for their former player if Arsenal were to agree on a sale to any other Spanish club, the Telegraph reports. The clause also indicates that Real Madrid must at least match the agreed upon transfer terms that Arsenal negotiates with the other Spanish club. Real also built in a whopping 33% of any profit of an Ozil transfer to a Spanish club if the final transfer fee is above €50 million, as well.
The leak of Ozil's contract and buyback clause follows the previously-secret transfer details surrounding the Gareth Bale and Anthony Martial moves which have opened up a door not only into how clubs and players agree and write up the terms of their transfers and contracts, but how there are extremely shady individuals who are quite dedicated to exposing the business that goes on within the sport.
Ozil's buyback clause nearly matches the clause that Arsenal and Barcelona agreed to when Cesc Fabregas moved back to Spain. And we saw how that worked when Arsene Wenger refused the first-option window to negotiate exclusively with Fabregas in the summer of 2014; Fabregas, as we all know, ended up moving to Chelsea a couple weeks later.
All of this said, all Arsenal need to do is not negotiate an Ozil transfer with another club in Spain. Considering there's only one club who'd be able to pony up the required transfer fee, FC Barcelona, I think it's safe to say that the clause was a further safeguard for Madrid to prevent them from losing a former star player of their to their hated rival.