@Bas ****
then just take the correct documentation needed to cancel the extension go get it cancelled and then leave.
Most people have the cancel documentation written to say that the day you are leaving the country is the day you want the extension cancelled :) AND they go about a week before they leave the country to do it. Immigrations will cancel the current extension and give you a new stamp dated until the date on the paperwork you turned in. (that way you're not cancelling the extension and then leaving the country that very day) :)