Importance of Canceling a Residence Permit?

I think you should talk to your employer peacefully and solve the problem. If you plan to leave, you should acquire the release letter from your employer so that you won't have trouble in looking for new jobs. You have to understand that your employer can't cancel your resident permit. However, he can report your situation to the PSB and then you can't work with your current one unless you renew your work permit and resident permit after you find a new job.

It doesn't matter whether you cancel it or not before you leave. As it is still valid, you can use it if you plan to enter PRC again. However, you can't work with it until you renew your work permit and resident permit after you find a new employer. This moment, the release letter is required.