The City Council may also authorize the issuance by the city of assignable certificates against abutting property owners or against persons, firms, corporations or estates using or occupying highways. Such certificates shall be payable to the contractor, or to the city, should the city pay the whole cost of improvements, as set out in Section 14, and the City Council shall fix the form and terms of such certificates. The recital in such certificates that proceedings with reference to such improvements have been made in compliance with the terms thereof, and that all prerequisites to the fixing of the lien and charge of personal liability evidenced by such certificates have been performed, shall be prima facie of the facts so recited, and no other proof thereof shall be required, but in all courts the said proceedings and prerequisites shall, without further proof, be presumed to have been had and performed. Such certificates shall be executed by the mayor and attested by the city secretary, or such other officer as may be designated by the City Council, with the corporate seal. Such certificate, when payable to the city, may be sold by it as may be provided by the City Council, and the proceeds of said sale shall become part of the permanent street improvement fund.