Employment Law Questions? Ask an Employment Lawyer.
They can reduce your salary without warning. As an at will employee (which you are unless you have an employment contract), you are subject to the whims of your employer in this regard. You can have your pay reduced on the spot, the same as you can be fired on the spot (or quit on the spot).
You would have a claim for any unpaid commissions, obviously. If you are owed compensation, and they have not paid, you could sue successfully for that.
One possible issue however is that you mention that you are starting a new branch. If you relocated for this position or had an expenses, you might be able to make out a claim under implied contract theory. In this case, the argument would be that you took this position with the understanding of a certain compensation... and because of that you have given consideration (left a job, moved, etc.). Under the implied contract theory, you might be able argue that you detrimentally relied on their promises and thus a contract was formed.
I have a formal offer letter signed and dated prior to my employment that outlines the details of my compensation package as far as the salary amount, health insurance contribution by the employer, vacation, commission rate and frequency of payment. But it does not mention a quota.
Does that mean anything?