If the board determines that the revenues from fees, charges, and rentals exceeds the amounts necessary for the proper maintenance, operation, repair and improvement of the parking places, and the acquisition of additional parking places, upon the order of the board the surplus revenue may be applied as a credit to each assessment in the proportion which the principal amount of each assessment bears to the total principal amount of all the assessments. The credit to each assessment, when added to all prior credits to that assessment, shall not exceed the principal amount of that assessment.