Visa Release/Transfer

I am currently teaching English at a training school in Shandong Jinan. I would like to leave my current position and plan to move to a new job. However, my current employer refused to give me any documents regarding my entry permit, to allow me to transfer the entry permit to my new employer.
I am leaving before the end of my contract, however I gave them more than the required 1 month notice. I get the feeling my school wants to hold onto my visa release form to prevent me from working in another school in China.
Do you have any information or advice re transferring my work Z?