Upon receipt of a denial of responsibility or upon the failure or refusal to make payment, the city finance director may cause proceedings to be instituted in any court of competent jurisdiction to collect the sum as a mature debt. Such proceedings shall be in addition to the lien provided by section 26-75 or any other remedy available to the city. Proof by the responsible party of the existence of an applicable exemption set forth in section 26-73 shall be a defense in any collection proceeding undertaken pursuant to this Code.