If everyone participating in a game agreed to it, then it was not unfair to any of them. That was "autonomy of will" in contracts.
It was like how, in Ancient Rome, marriages between men and women were arranged by their fathers. Once the fathers reached an agreement, the children had to marry no matter what they thought. By Justinian's time, the woman could represent herself in making a contract with the man, though that contract might contain certain "little tricks."
Goblins would not lightly enter into "any" contract with humans, much less form magical contracts like House-elves. He had merely reached an agreement with Harry: he would provide assistance, and then the Sword of Gryffindor would be given to him. The entire Goblin world had been shaken by the matter. If Griphook truly brought that sword back, he would become the new "Goblin King."
As long as both parties acted voluntarily, even if outsiders considered it unfair, outside forces could not interfere, just as they could not interfere with the parties "voluntarily" entering into a contract.
Then how could fairness and justice be guaranteed? Both "consent" and "willingness" could be manufactured. Take the Treaty of Aranjuez in 1801, for instance. The treaty clearly stipulated restoration, not retrocession.
The New World was virgin land, originally untainted. Every year, Hera went to the Fountain of Youth to wash away her age and restore her virginity. That was restoration.
Retrocession carried the meaning of returning something. It was like a trust: the settlor transferred land to a friend and had him use the proceeds to care for his family. The friend did so, and when the settlor returned, the land was handed back to him.
Even setting aside the issue of the recipient of the grant and the transfer of real-property ownership,