What should I do, given that I sold a plot of land more than twelve years ago and handed over the documents to the buyer, but the sale was not formally completed, and I discovered in 2013 that it is still in my name, knowing that the buyer passed away more than ten years ago and I do not know of any heirs for him?

If the sale between you and the buyer was a genuine transaction, then the land belongs to the buyer and devolves to his heirs after his death, even if it has not been officially registered.

If you have already handed over the land and documents to the buyer, you have fulfilled your obligation. However, if you have not yet handed them over, you must search for the heirs and deliver the land and documents to them, as failing to do so may prevent them from benefiting from it.

If you cannot find any heirs, the land takes the ruling of property whose owner is unknown (مال مجهول صاحبه), and it should be disposed of in the ways designated for ownerless property. If there is an official body or a law that mandates its surrender to the state, then it must be handed over to them. Otherwise, it should be given as charity on behalf of its owner.