I just quit my job. The company wants to cancel my Z visa, BUT I'm not ready to return to the US. Can they cancel the Visa if I don't give them my passport?

I just quit my job. The company wants to cancel my Z visa, BUT I'm not ready to return to the US. I want to stay in China another month or so to say goodbye to my friends and visit some landmarks. Can they cancel the Visa if I don't give them my passport? What are my best options?