Fired!

Hi all, I have a question. I have recently been fired (for a number of reasons, none of which I agree with). Those details are irrelevant but they have finished my contract one month early. I have a Z and a foreign expert cert (which I have never seen) but they are saying they need my passport to cancel my Z and if I delay it could affect the cancellation of my FEC and my chances of getting another job in China. I don't want to give them my passport as it's far too valuable to hand over to someone who, quite frankly, I do not trust. I want to do my last month in peace and let my Z expire naturally. Can I do this? Do they need my passport to cancel my FEC? and if they don't cancel it will it affect my future. I will leave in May and come back hopefully in August so by then most things should have expired naturally right? What are my options?