A discrepancy between your Balance Sheet and your Accounts payable ageing report could be the result of future payments made against current bills.
The Accounts payable ageing report, like other Accounts payable detail reports, doesn't include bills with a status of Paid, so, while they may seem to disagree, both the Balance Sheet and the Accounts payable ageing detail reports are correct because the bill is no longer open.
If necessary, you can generate a Supplier Balance report that agrees with the Balance Sheet. To do this, customise the Supplier Balance report to run as of the date on the Balance Sheet, and change its Ageing method to Report date.
Alternatively, you can customise the Accounts payable ageing report to the as of Report date. The report will then agree with the Balance Sheet as long as the Balance Sheet follows the accrual reporting method for the same date range.
For more information: Follow these steps to match mismatched Ageing Reports in QuickBooks Online.