This is the sort of thing for which you hire an attorney. If they kick you out for the good of the service, they can't just send you a bill and expect payment (well, they could, but they are inviting a fight.) First of all, you could easily debate the value of what they have given you, especially if you have not completed your degree and there is a question whether you have anything with which you can or will complete a medical degree. Medical school obviously isn't like undergrad or graduate school with transfer of credit being a common procedure. Second, if they are creating the circumstances for your discharge, their OER by their evaluators, and they have not afforded you an opportunity at remediation and cure of whatever defect they claim exists, then you would be within reason to refuse to pay them. And you having received the pay of an O-1 is not something they have any reasonable right to see returned either; companies that fire employees don't get to claim return of paid wages.