Whenever the revolving fund provided for in this article after payment of all bills due against a district exceeds twenty thousand dollars ($20,000) or exceeds such lesser sum as the board of directors shall determine to be a sufficient working fund for the purposes of this article, the board shall apportion such surplus to be repaid to the counties forming the district, in the same proportion in which they are required to contribute to the revolving fund in the first instance, the payments to go into the county general funds of such counties.