Nothing in this act shall be taken to prohibit a corporation or person engaged in the production, generation, transmission or furnishing of heat, light, water or power, or telephone service, from establishing a sliding scale of charges: provided, that a schedule showing such scale of charges shall first have been filed with the commission and such schedule and each rate set out therein approved by it. Nothing in this act shall be taken to prohibit any such corporation or person from entering into an arrangement for a fixed period for the automatic adjustment of charges for heat, light, water or power or telephone service, in relation to the dividends to be paid to stockholders of such corporation, or the profit to be realized by such person: provided, that a schedule showing the scale of charges under such arrangements shall first have been filed with the commission and such schedule and each rate set out therein approved by it. Nothing in this section shall prevent the commission from revoking its approval at any time and fixing other rates and charges for the product or commodity or service, as authorized by this act.