I'm happy to help and guide you on how to correct the balance forward in the Account Summary.
First you'll need to determine which transactions are incorrectly posting to show this balance forward on your invoice. Here's how:
On the left tab, click Sales.
Head to the Customers section.
Click on the customer's name who has the incorrect invoice Balance Forward.
From the New transaction drop-down arrow, select Statement.
Select the Balance Forward option on the Statement type drop-down arrow.
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 invoice 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.