California Code, Labor Code - LAB § 224

The provisions of Sections 221, 222 and 223 shall in no way make it unlawful for an employer to withhold or divert any portion of an employee's wages when the employer is required or empowered so to do by state or federal law or when a deduction is expressly authorized in writing by the employee to cover insurance premiums, hospital or medical dues, or other deductions not amounting to a rebate or deduction from the standard wage arrived at by collective bargaining or pursuant to wage agreement or statute, or when a deduction to cover health and welfare or pension plan contributions is expressly authorized by a collective bargaining or wage agreement.

Nothing in this section or any other provision of law shall be construed as authorizing an employer to withhold or divert any portion of an employee's wages to pay any tax, fee or charge prohibited by Section 50026 of the Government Code, whether or not the employee authorizes such withholding or diversion.