Is it permissible, in Islamic Law, for a person to dispose of lands registered in his name in the real estate registry by his grandfather, after he paid the cost of their mortgage redemption from his own money?

The mere registration of land by a grandfather in his grandson's name does not prove the grandson's ownership of the land, because it is not a sale and is not considered a gift. Even if it were a gift, a gift is not effective unless the donee takes full possession of the gifted item before the death of the donor; otherwise, it becomes part of the deceased's estate. Therefore, the grandson is not permitted to dispose of these lands unless the gift is proven and he legally possessed them before his grandfather's death. The grandson's payment of the mortgage price does not entitle him to dispose of them; rather, it entitles him to claim back the price he paid from the estate, if he paid it with the intention of claiming it back and can prove this with evidence or with the acknowledgment of the heirs.