Cal. Rev. & Tax. Code § 60521.5

When an amount represented by a person who is a taxpayer under this part to a customer as constituting reimbursement for taxes due under this part is computed upon an amount that is not taxable or is in excess of the taxable amount 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 taxable or is in excess of the taxable amount, 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 transaction 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.