You would need an attorney to review the offer letter to determine if it can be considered a legal contract. Without a contract an employer can change your rate of pay without notice as long as you are paid at least minimum wage.
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The first question I would have was there an employment contract in place outlining the conditions of your employment. If so one must look to this agreement as what can and can't be done. Absent an employment agreement I would need to know if you were employed in an at will state? Seeking the advice of a qualified attorney would be very helpful to you.