I've got just the report to help you figure out this customer balance issue. The only way to correct a Balance Forward is by determining which transactions are incorrectly affecting the amount. You can accomplish this with the instructions detailed below:
Verifying a Customer Statement
From QuickBooks Online, navigate to the Sales tab and select the Customers section.
Click on the customer's name who has the incorrect Balance Forward.
From the New transaction (▼) menu, select Statement.
Within the statement: make sure the type is Balance Forward, select a Start Date that occurs before the customer's first transaction, and click Apply.
The customer's current Balance Forward will appear and should align with what's appearing on the invoice. You can click Print or Preview to see a full list of those transactions. Once you've determined which items need changed, click the X to close the statement and find the transaction in the customer's list. Use the Filter menu to quickly find an item if necessary.
Check out this brief video demonstration of a customer statement displaying a credit balance:
With these steps, determining exactly why the customer is showing a credit balance will be a breeze. This information is also available from our guide on creating balance forward statements here. Please know you can always find me here for all of your QuickBooks needs. Thanks for reaching out, wishing you continued success in all of your future endeavours.