The Customer Balance field in the Quotes, Sales Orders,Sales/Invoicing, and Credit Memos windows displays the total of all unpaid invoices currently in the customer ledger as of the system date. Even if you are in a previous accounting period and your system date is today's date, the customer balance reflects all unpaid invoices as of today's date.
If you select the customer balance link, the current Customer Ledgers report for this customer will appear.