Upon the completion of any improvement the City Auditor shall rebate to the then owner of the property which shall have been assessed to pay for such improvement, any surplus or excess remaining unexpended for the purpose for which such assessment was made, and in the event of there being a deficit in the fund provided for the making of any such improvement the City Auditor shall report to the Council a supplementary assessment within the limitations hereinbefore provided, which supplementary assessment shall be made by ordinance and certified for collection as is provided in the case of original assessments.