Once you have your PR, your employer no longer has any say in it at all (unless you are convicted of a criminal offense against the employer then there is a good possibility that the government will cancel it). But under normal circumstances if you resigned they can't do jack! They might try to threaten but there is nothing really that they can do.
SOME PEOPLE TRY TO TURN BACK THEIR ODOMETERS. NOT ME. I WANT PEOPLE TO KNOW WHY I LOOK THIS WAY. I'VE TRAVELED A LONG WAY, AND SOME OF THE ROADS WEREN'T PAVED. ~ Will Rogers