This is called "the sin of omission." Pretty sure when we mention the many sins and transgressions we have made during Yom Kippur tefilot, that's on the list.
Also, they have told you a specific price that must be paid, upon asking if it's okay to extend another week (same amount as first week). You have been informed that your action of extending the rental by one week is acceptable based upon the consideration that you will pay the original price again at the end of the extension.
You have entered into an agreement and broken your word.