What am I entitled to upon termination?

You should consult with an employment law attorney. Contracts of employment, including those formed by offer letters, are enforceable; however, when an employment contract does not have a period of time which it covers set or specified in it, the employer may be entitled to change or reduce salary at some future point.  Therefore, depending on exactly what the offer letter/letter of employment says, how long you have worked here, whether your hours or jobs have changed from what they were originally to be, etc., the employer may have been able to pay you less than $52k/year--or alternately, the employer may still have been obligated to pay you at that rate, which could give you a cause of action for the unpaid wages. You need an attorney to review the situation and the letter in detail, to determine your rights.