Real Madrid has a buy-back clause in their transfer agreement with Arsenal for Mesut Ozil, according to leaked transfer documents. The contract on the Football Leaks website shows that Arsenal must notify Real Madrid if they ever intend to sell the World Cup winner to another Spanish club. The clause would give Real Madrid 48 hours to decide if they match the offer. The option would not apply if Arsenal decided to sell the German player outside of Spain. In this case, Real Madrid would have to least the deal that they had agreed on elsewhere.
http://www.timeslive.co.za/thetimes/2016/01/26/Arsenals-R892-million-Mesut-Ozil-deal-is-revealed