Cal. Rev. & Tax. Code § 41023

The surcharges required to be collected by the service supplier, and any amount unreturned to the service user that is not owed as part of the surcharges but was collected from the service user under the representation by the service supplier that it was owed as part of the surcharges, constitute debts owed by the service supplier to this state.

A service supplier that has collected any amount of surcharges in excess of the amount of surcharges imposed by this part and actually due from a service user, may refund that amount to the service user, even though the surcharge amounts have already been paid over to the department and a corresponding credit or refund has not yet been secured. The service supplier may claim credit for that overpayment refund against the amount of surcharges imposed by this part that is due upon any other return, providing that credit is claimed in a return dated no later than three years from the date of overpayment.