40 ILCS 5/5-168.1
The employer may pick up the employee contributions required by Sections 5-167.1, 5-169, 5-170, 5-171 and 5-175.1 for salary earned after December 31, 1981. If employee contributions are not picked up, the amount that would have been picked up under this amendatory Act of 1980 shall continue to be deducted from salary. If employee contributions are picked up they shall be treated as employer contributions in determining tax treatment under the United States Internal Revenue Code; however, the employer shall continue to withhold Federal and state income taxes based upon these contributions until the Internal Revenue Service or the Federal courts rule that pursuant to Section 414(h) of the United States Internal Revenue Code, these contributions shall not be included as gross income of the employee until such time as they are distributed or made available. The employer shall pay these employee contributions from the same source of funds which is used in paying salary to the employee. The employer may pick up these contributions by a reduction in the cash salary of the employee or by an offset against a future salary increase or by a combination of a reduction in salary and offset against a future salary increase. If employee contributions are picked up they shall be treated for all purposes of this Article 5, including Section 5-168, in the same manner and to the same extent as employee contributions made prior to the date picked up.
40 ILCS 5/5-168.1