S.C. Code Ann. § 41-7-40

Nothing in this chapter precludes an employer from deducting from the wages of the employees and paying over to a labor organization, or its authorized representative, membership dues in a labor organization; however, the employer must have received from each employee, on whose account the deductions are made, a written assignment which must not be irrevocable for a period of more than one year or until the termination date of any applicable collective agreement or assignment, whichever occurs sooner. After one year, the employee has the absolute right to revoke the written assignment allowing for deduction of membership dues in a labor union.