Retro-active pay cut?

Is it illegal to make a pay cut retroactive?

Long story short: I work in the corporate office of an IT staffing company. My manager get's paid by the client about three months after the consultant, and recently he found out that the client initiated a pay cut three months ago for 10%. The total dollar amount is $3234 or 103 hours at the consultants new rate.

My manager is planning to pass this cut retroactively to the consultant by deducting hours from his paychecks until the difference is made up.

The consultant is screaming mad, and threatening to sue. I agree with him, but I don't know if there's much I can do to stop my boss from initiating this cut. We've done it to another consultant, I just didn't know at the time that it was illegal.