Just for information: In my local Labour Office, they will not allow the work permit to be cancelled unless it is done by the employer. They will also not allow it to be cancelled in advance of the termination of employment date. It must be done on the actual termination of employment date, which obviously means you can't cancel your extension of stay based on work at Immigration in advance either, since they require the receipt from the Labour Office.
So, different provinces, different rules, which make things that should be simple, that bit less so. :)
So, I guess a trip to the local immigration office is necessary, where I will probably get different answers from different officers, like I did before I went to get my current extension on April 3.