You cannot go back in history.
If you already used the credit since you had a 80 dollars service that this credit is gone
By deleting the database service (or any other service) more credit will not be used
So, payment is not canceled for what you used already, but once you delete the database payment is stopped at the point it was when you deleted the database
THIS IS IMPORTANT POINT WHEN USING CLOUDE SERVICES: You PAY ONLY FOR WHAT YOU USE. stop using will not cancel previous payment for what you already used but will stop future payment for what you will not use