Real Madrid have a buy-back clause in their transfer agreement with Arsenal for Mesut Ozil, leaked transfer documents have claimed.
A contract on the Football Leaks website claims 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 international would have 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 €50m (£38m), Real would be entitled to 33 per cent of the profit.
The contract, which is unverified, shows Arsenal paid an initial fee of €44m (£33.5) when they signed Ozil in September 2013, with the fee rising to a maximum of €50m (£38m).
Six separate €1m (£760,000) payments, up to a maximum of €6m (£4.5), are contingent on Arsenal qualifying for the Champions League group stages in any season from July 2014 onwards.
Each payment must be settled within 30 days of qualification being secured, the contract states.
Arsenal declined to comment.