What is the ruling on reclaiming agricultural land that I inherited, but was sold to another person by someone claiming to have bought it, knowing that the person who gifted the land to my father is still alive, and is his sale of the land permissible after he gifted it to my father?

In the present time, ownership is established by registering properties with the competent government departments, and this registration takes the place of proven possession for ownership. From a Sharia (definition) perspective, contracts become binding merely by their utterance, provided they fulfill the Sharia conditions. So, if a person gifts a farm to another, and the donee takes possession of it, the donor is not permitted to reclaim it, and it becomes the property of the donee. The judiciary is not permitted to alter this, and if it does, the sin is upon whoever took the right knowing that they were not entitled to it.