Any funds arising from assessments made under this part remaining in the county treasury after the payment of all outstanding legal obligations incurred by the district, shall be ordered transferred by the governing body to the city treasurer of any city that has been incorporated since the formation of the district, and which includes the district or a considerable portion thereof within its corporate limits. If no such city has been incorporated then the funds so remaining in the county treasury shall be transferred to a separate fund and upon the order of the governing body shall be repaid pro rata to the persons by whom the assessments were originally paid.