This important change for the users seems to be stealthily implemented has not been clearly explained by Unity, and many users here have the trouble, so I would alert here. Previously, it was explained by Unity that remaining budget of advertisement will be refunded. You can see some explanation in this forum as such. However, recently (I do not when but between 2 years ago to 5 months ago), they have changed the terms, and for some reason, they decided not to refund. Also, what I have experienced is, not only the important change was not noticed, Unity has not even responded to any of tickets and email requesting refunding. I am posting this post to prevent any other users that will become the victim again. Be careful about depositing advertisement fund, and be careful about Unity's stealth change of the terms.