Compensation to be paid the city for said city lands shall be paid only from contributions which may be made to the city for that purpose or from revenues theretofore or thereafter derived from said city lands or other lands acquired for the district as parking places. The right of the city to compensation from said revenues may be made subordinate to the rights of the holders of any bonds thereafter to be issued by the city under this part for the district; provided, however, that if at the time of adoption of the ordinance there are outstanding bonds issued by the city under this part for the district, the right of the city to compensation from said revenues shall be subordinate to any rights of the holders of such bonds to such revenues.