I've heard that before the annexation of the Byzantine Empire by the Ottomans, that the last Byzantine Emperor sold the title to Ferdinand II of Aragon. So the current Spanish king could theoretically claim the lands of the Byzantine Empire, right? Is this true or just some misconceptions?
Andreas Palaiologos, who was a grandson of Manuel II, was living in Rome and, in need of money, sold his title to the monarchy of Spain. The title eventually fell out of use, it was no longer really all that prestigious and sort of faded away much like the title King of Jerusalem. The Empire hasn't existed for almost 600 years, the King of Spain could lay claim to all he wants but nobody would recognize it.