Compensation and Overtime: Can an Employer Reduce Commissions in One Category Due to Performance in Another

I earned 20% commission in category A but since I did not hit my goals in category D my employer is threatening to reduce my commissions in category A by 2%. which is ofcourse the most lucrative category and where most of my earnings are derived.

Is this legal/State of Illinois ?

Compensation and Overtime: Can an Employer Reduce Commissions in One Category Due to Performance in Another