Nothing in this subchapter shall be construed to limit the authority of the Secretary to acquire land and interests in land within the boundary of any System unit. The Secretary shall give prompt and careful consideration to any offer made by the owner of any valid right or other property in Glacier Bay National Monument, Death Valley National Monument, Organ Pipe Cactus National Monument, or Mount McKinley National Park to sell the right or other property if the owner notifies the Secretary that the continued ownership of the right or property is causing, or would result in, undue hardship.