California Code, Government Code - GOV § 23008

Whenever it is economical and satisfactory to do so, a county may lease equipment, perform work, or furnish goods for any district or municipal corporation within the county, if before the work is done or the goods are ordered or furnished by the county, an amount equal to the cost, or an amount 10 percent in excess of the estimated cost, is so reserved from the funds of the district or municipal corporation to be charged that it may be transferred to the county, when the work is completed or the goods are supplied.