Employee leaving early

I am a dentist employed by the government in a clinic.

According to my contract, I am employed to work from 8am to 4pm. However, my supervisor sometimes tell me that I can go at 11 am. He leaves early and also tells me that I can leave early.

Although he is my supervisor, I do not know whether he has the authority to let me go early, as he is not my employer. Rather, I am employed by the department.

Is it permissible for me to leave early? If I do so, will my income be halaal?

Sometimes but not always, I am forced to leave early as the supervisor locks the clinic and leaves, so I am unable to attend to patients even if I remain. What should I do in such a case?