Section 56B. All contracts made by a municipal light commission where the amount involved is five thousand dollars or more shall be in writing. Any contract made as aforesaid may be required to be accompanied by a bond with sureties satisfactory to the municipal light commission, or by a deposit of money, certified check or other security for the faithful performance thereof, and such bonds or other securities shall be deposited with the city or town treasurer until the contract has been carried out in all respects; and no such contract shall be altered except by a written agreement of the contractor, the sureties on his bond, if any, and the officer or members of the municipal light commission making the contract.