Welcome to the Xfinity Forum, and thanks for your question, @nandos707. A negative balance on your account means you have a credit balance. If this happens in the future, you can cancel a payment by following these steps, Managing or Canceling Scheduled Payments.
Managing or Canceling Scheduled Payments
Click Manage Scheduled Payments under the Billing tab.
If you need additional assistance with the payments on your account, feel free to send me a private message, and we'd be happy to help! To send a message, click on my name "ComcastChe," then click send a message. Please include your first and last name and service address.