Each city of the first class now or hereafter having a population of 50,000 inhabitants or more, including each such city operating under a charter adopted pursuant to the provisions of the Constitution of the state of Minnesota, article IV, section 36, article XI, section 4, or article XII, section 5, is hereby authorized and empowered to acquire by purchase, condemnation, or otherwise any right or interest in land either platted or unplatted within the limits of the city, which interest in land consists of a right or privilege in the owner of the land to offset certain amounts against special assessments levied by the governing body, the city council, or the board of park commissioners of such city for park or parkway purposes, or both.