Real Madrid have a buy-back clause in their transfer agreement with Arsenal for Mesut Ozil (right), according to leaked transfer documents.
A contract on the Football Leaks website shows that the Gunners must notify Real if they ever intend to sell the World Cup winner to another Spanish club.
The financial terms of the offer received for the Germany playmaker would be disclosed to Real, who would have 48 hours to decide whether to match the offer, or until noon, London time, on the final day of the relevant transfer window, whichever comes first.
Should a rival Spanish club sign Ozil for more than €50 million, Real would be entitled to 33 per cent of the profit.
The contract shows Arsenal paid an initial fee of €44m when they signed Ozil in September 2013, with the fee rising to a maximum of €50m.
Six separate €1m payments, up to a maximum of €6m, are contingent on Arsenal qualifying for the Champions League group stages in any season from July 2014 onwards.
Each one must be paid within 30 days of qualification being secured, the contract states.
An Arsenal spokesman said: "We don't discuss contracts of any of our players."