Your employer has no right to ask for you to reimburse the company for insurance premiums that have already been paid. If you were part of a legitimate Small Business policy, and your reduced hours did not affect your eligibility to remain under the coverage, then you are not liable. Typically, when an employee is taken to part time employment, they will loose their eligibility to remain insured under the group policy. They may elect to go on COBRA, at which time the employee becomes responsible for the payment. However, if this had been the case, then the insurance company would have billed you directly. More than likely, your employer did not inform the insurance company of your part-time status. Your response to your employer should be to ask him to prove that you are liable for this back payment.  He will not be able to successfully do so.