Your cash balance is found in your Billing Summary (see blue box in image below).
If you upgrade, downgrade, remove numbers etc., you might have a cash balance. This is because we now prorate accounts. To see past information, click on View History. The Cash Balance History table will appear. If you have a history, you will see the transactions.
You can use your cash balance for any one-time payment when manually applied. For example:
adding bucket of credits
adding a new number
upgrading/downgrading /making changes to your plan
For more information about billing, check out this helpdesk article.