California Code, Penal Code - PEN § 4004.5

(a) A city may furnish facilities to be used for holding prisoners held for examination or during trial without cost to the county or upon such terms as may be agreed upon by the governing body of the city and the board of supervisors, and the marshal may keep the prisoners in their custody in the city jail.

(b) A city may furnish facilities to be used for holding persons convicted of a public offense who have been transferred from the county jail by the sheriff due to crowded conditions upon those terms as may be agreed upon by the governing body of the city and the board of supervisors.  The agreed terms may indicate that the facilities are to be provided free of charge to the county.