50 ILCS 125/2
Any local governmental agency, unless provided otherwise in any statute specifically creating or governing any such agency, may by ordinance or resolution of its corporate authority authorize the withholding from the compensation of employees the union dues of such employees payable to any labor organization and membership dues of such employees payable to professional organizations upon the written request of the employee, which the employee may revoke in writing at any time. The appropriate officer of the local governmental agency shall pay over the amounts withheld to the treasury of the union or professional organization at the times specified in the ordinance or resolution of the corporate authority of such local governmental agency.
50 ILCS 125/2