However, Ashipashazade's phrasing also suggests that these fortresses remained contested with the Byzantines thereafter, and not until the reign of Mehmed II (r. 1444–1446, 1451–1481) were they definitely conquered by the Ottomans.[16][17] If Timur should attack Constantinople, Süleyman undertook to help defend it with his galleys and sailors.[11] Süleyman also returned the islands of the Northern Sporades: Skopelos, Skiathos, and Skyros. Their tribute was likewise annulled.[18] All citizens of Constantinople (i.e., of the Byzantine emperor) were allowed to return to their homes without any impositions.[18] All cases of litigation from the time of Süleyman's father and grandfather were dropped, except for cases of debts between individuals.[18] The Serbian ruler Stefan Lazarević was to be allowed to retain his lands, on the condition that he accepted the same obligations owed to Bayezid, i.e. pay tribute and provide military assistance.[18] All Frankish, Venetian, Genoese, Rhodian, and Greek merchants were free to trade in any territory possessed by Süleyman then or in the future, and would be obliged to pay in duties "what was usual before".[18] If a merchant commits a crime, no other merchants should be punished other than the perpetrator.[18] If a shipwreck should occur in Süleyman's territory, both goods and passengers would be returned.[18] All ports under Süleyman's control would be open to Christian merchants, who would be allowed to export grain without restrictions.