Does a Nurse Have to Pay Back a Sign-On Bonus?

A nurse signed a contract to work the night shift on weekends at a local hospital and received a $10,000 sign-on bonus. Shortly after beginning orientation, management gave her shift to someone else and told her to work the day shift five days a week. After two months, the nurse quit her job and moved out of state. Subsequently, the hospital asked her to repay the $10,000 bonus. Is the employer’s request legal?