Do anything you can to avoid being shorted...because the only way you will solve it after-the-fact is in court.
It is not legal for them to hold your money for any reason, just FYI. If your commission gets paid monthly, there should be no reason for you not to get your check just because you didn't work the entire month. And even if they claim a 'legitimate' reason, like you being in possession of company property (be it their product, uniforms, tools or equipment, ect)...they still have no right. You get your money, and it's their job to pursue anything they feel you may owe them in return.
Unfortunately, we live in a shithole, and the agency that is supposed to enforce those kinds of things does absolutely nothing. Hence the bit about having to sue them if things don't go well.