(a) If it appears to the Director that any agreement, understanding or arrangement exists between any persons which causes any person’s base income allocable to this State to be improperly or inaccurately reflected, the Director may adjust such items of income and deduction, and any factor taken into account in allocating income to this State, to such extent as may reasonably be required to determine the base income of such person properly allocable to this State.
     (b) The Director may not make an adjustment to base income under this Section that has the same effect as retroactively applying any amendments to this Act made by Public Act 93-0840, Public Act 95-0233, or Public Act 95-0707.