Upon the completion of any improvement, the Finance Director shall rebate to the then owner or owners of the property which shall have been assessed to pay for such improvement, any unexpended surplus. In the event of a deficit in the fund provided for such improvement, the Finance Director shall report to the Council a supplementary assessment within the limitations herein provided for, which supplementary assessments shall be made by ordinance of the Council and certified for collection as is provided in the case of original assessments.