What is the ruling on purchasing a real estate receipt for a property not yet taken possession of, then selling this receipt to more than one person, so that it can be claimed after thirty years, while the land remains in the seller's possession?

It is impermissible to sell an item before taking possession of it, whether it is a movable good or real estate. This is based on the Prophet's (peace be upon him) saying: "Whoever buys food should not sell it until he has taken full possession of it," and his (peace be upon him) prohibition "that goods be bought where they are bought until merchants take them to their dwellings." Therefore, the aforementioned real estate still belongs to its first owner, and the sales transactions that took place concerning it are invalid. Those involved in these transactions are entitled to reclaim what they have paid.