@Karl ******
take your termination paperwork to the immigration office where you got your extension inked in and see if you can cancel it, most offices let you do it a few weeks before the actual termination date on the paperwork and they stamp you in until that date (so you don't have to leave the day you cancel it)
BUT
Some offices make you cancel it effective that day and then you have to buy that 7 day emergency stamp to get out of thailand in a week.
You may or many not be required to show your work permit is cancelled, that's totally up to your immigration office