ubi, employer/employee relationship is a contract, but not of equals. Halakha recognizes it. Employee has an aspect of avdut as employer control employee’s time. So, employee is allowed to quit without prior notice. There might be an issue of damages if someone is not done on time or spoils, but the employer should let employee go even if this causes inconvenience in order not to enslave him.
So, it should be possible for employee to consider another job. On the other hand, you should be sensitive and not to take away a job from a Jew who is already working, unless it is a rare opportunity that is important for you.