At any time, either before or after the formation of the district, the legislative body may provide by ordinance that, for a period specified in the ordinance but not exceeding 25 years from its operative date, all or any specified amount, portion or percentage of revenues derived by the city from parking meters on streets within the district shall be used for the purposes specified in the ordinance which may include the acquisition, improvement, maintenance, repair, and operation of parking places for the use and benefit of the district. The purposes specified in the ordinance may also include the application of such revenues as a credit upon the levied assessments in the same manner as is provided in Sections 31828, 31829 and 31830. From the operative date of the ordinance such revenues shall for the period specified be placed in such fund or funds of the district as the board of commissioners specify and may be expended only for the purposes specified in the ordinance.