§ 173-f. Every terminable permit, and every amendment to any existing license, grant, franchise, permit or consent incorporating therein the terms of a terminable permit, shall contain an irrevocable option for the purchase by the city, town or village, either directly or through its nominee or nominees, of all or any part of the property, plant and equipment of the street surface railroad at any time, at a price agreed upon or to be ascertained and determined in a manner agreed upon at the time of the granting of the permit or amendment and to be set forth therein. Any such permit or amendment may contain provisions for amortizing out of earnings or in accordance with an amortization schedule or schedules to be set forth therein, over a period not exceeding fifty years, the price to be paid for all or such part of the property, plant and equipment of the street surface railroad and for any additions thereto or betterments or improvements thereof, so that upon the exercise of the option the city, town or village, or its nominee or nominees, shall be required to pay only the unamortized portion of such price, and may contain any other terms, agreements, stipulations and conditions as may be authorized by any statute of the state, or as the city, town or village may deem proper and in the public interest, and may provide for the termination thereof for any legal cause.