§ 21-e. Operating assistance for water-works corporations in certain cities. Each city having a population of more than one million may provide operating assistance to a water-works corporation subject to the provisions of the public service law which provides water service in such city for the purpose of reducing or eliminating disparity between rates charged for such service by the municipal water supply system of such city and rates charged by such water-works corporation, and in furtherance of such purpose the chief executive officer of such city may enter into an agreement with such water-works corporation and expend such sums as may be appropriated therefor so as to effect a reduction of rates of such water-works corporation.