Cal. Rev. & Tax. Code § 50139.5

When an amount represented by a person who is a feepayer under this part to a customer as constituting reimbursement for fees due under this part is computed upon an amount that is not subject to that fee or is in excess of that fee amount due and is actually paid by the customer to the person, the amount so paid shall be returned by the person to the customer upon notification by the State Board of Equalization or by the customer that the excess has been ascertained. If the person fails or refuses to do so, the amount so paid, if knowingly or mistakenly computed by the person upon an amount that is not subject to the fee or is in excess of the fee due, shall be remitted by that person to this state. Those amounts remitted to the state shall be credited by the board on any amounts due and payable under this part on the same activity from the person by whom it was paid to this state and the balance, if any, shall constitute an obligation due from the person to this state.