California Code, Government Code - GOV § 16585

(a) A city, county, or city and county may sell or transfer part or all of its accounts receivable to a private debt collector or private persons or entities, provided the city, county, or city and county notifies the debtor in writing at the address of record that the alleged accounts receivable debt will be turned over for private collection unless the debt is paid, or appealed within a time period, as determined by the city, county, or city and county providing the notice.

(b) No city, county, or city and county shall assign or sell any account receivable pursuant to subdivision (a) if the account receivable debt has been contested.