Is the money taken by the tenant in exchange for waiving rights to the land considered unlawful (haram)? If so, should the price of the land be returned at its value at the time it was taken, or at its value today?

This money you took is unlawful, and it is not permissible for you to take it, because it is based on falsehood, as the law issued concerning it contradicts Islamic Sharia (definition). This is because the lessor is the true owner of the leased property, and the lessee only owns its usufruct for the duration of the agreed-upon contract. Therefore, you must return what you took to its owner. If the land is still in your possession, you must return the land to him unless he sells it to you or willingly grants it to you as a gift.