Is what a real estate broker does considered a bribe if he helps a land seller sell it to a person belonging to the ruling family for a large sum, knowing that the seller was previously prevented from disposing of his land?

It is permissible to work as an intermediary (broker) and receive a fee for that. It is also permissible for the landowner to pay a sum to someone who helps him sell it, even if that is in exchange for recovering his right. However, this intermediary is not permitted to take money for mediating the restoration of a right unless he expends effort or spends money, in which case he may take an amount proportionate to that. If taking money is prohibited, it is permissible to give it to reclaim a right or repel injustice.