§ 131. Letting of contract to be public. Every such contract shall be let at public letting as required by law, and as a consideration for the execution and performance thereof, shall expressly provide for and secure to such city, prices lower than any now prescribed therein by law, and a progressive reduction in such price for each year during the continued performance of such contract and also adequate assurance of the continuing mutual performance of such contract according to the conditions thereof, with proper indemnity to either party to such contract against any possible violation, impairment, abrogation or supersession thereof, within the term specified.