What can we do? We overpaid part-timer, who knew it and didn’t come forward

Q. Because of a human error in our payroll department, one of our part-time employees has been receiving a full-time salary for the past six months. Although we recently realized our mistake, the employee never came forward to let us know about the error. Aside from recouping the funds, what actions, if any, can we take against this employee for not being forthcoming?