N.Y. General City Law § 131

§ 131. Letting of contract to be public. Every such contract shall be\nlet at public letting as required by law, and as a consideration for the\nexecution and performance thereof, shall expressly provide for and\nsecure to such city, prices lower than any now prescribed therein by\nlaw, and a progressive reduction in such price for each year during the\ncontinued performance of such contract and also adequate assurance of\nthe continuing mutual performance of such contract according to the\nconditions thereof, with proper indemnity to either party to such\ncontract against any possible violation, impairment, abrogation or\nsupersession thereof, within the term specified.\n