Cal. Rev. & Tax. Code § 41023

The surcharge required to be collected by the service supplier, and any amount unreturned to the service user which is not a surcharge but was collected from the service user as representing a surcharge, constitute debts owed by the service supplier to this state.

A service supplier that has collected any amount of surcharge in excess of the amount of surcharge imposed by this part and actually due from a service user, may refund such amount to the service user, even though such surcharge amount has already been paid over to the board and no corresponding credit or refund has yet been secured. Any service supplier making a refund of any charge to a service user upon which surcharge is collected under this part from the service user may repay therewith the amount of the surcharge paid. The service supplier may claim credit for such overpayment against the amount of surcharge imposed by this part which is due upon any other quarterly return, providing such credit is claimed in a return dated no later than three years from the date of overpayment.