Section 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.