I'm not sure what you mean by "withhold". Is there simply a lack of payment? Or has the employer acknowledged the amount due and applied it to some amount the employer believes you owe to the employer?
The answer to your question will be determined based upon your written agreement with the employer or, if there is none, upon the written policies on commission payments that the employer has implemented. Without a specific policy allowing the "withholding" of commissions, however, it's a pretty tough task for an employer to "withhold" commissions without incurring liability.
There are instances in which employers simply run out of money without there being in bad motive. Employees tend to have a higher priority for payment then many other creditors if there's ever a bankruptcy filing by the employer.
This isn't legal advice. I'm just trying to provide some background information so you can think about this situation and have a more meaningful conversation with an attorney in the future.