@Luke **********
> so technically we would be in overstay if we cancel on Friday?
A person would leave Thailand, technically, when they cross immigration. If you cross immigration by 11:59PM Friday evening, you will have left Thailand, even if you are waiting in a waiting area for your flight.
However, talk to your company or immigration office. Some immigration offices allow the extension of stay to be cancelled, post dated (for example, you may be allowed to cancel your extension of stay on Monday, but have the "admitted to" date changed to your flight, for example, October 1).