After the bonds and all interest thereon have been fully paid, or prior thereto to the extent permitted by express provision of the ordinance, resolution, or indenture providing for the issuance of the bonds, all revenues derived from the operation of the parking facilities and not required for the operation and maintenance of the facilities may be used for the further improvement of the facilities for the benefit of the district, as the parking place commission of the district may decide. Any excess revenues shall be paid into the general fund of the city or the fees and charges reduced so that there will be no excessive revenue.