Determining unpaid invoices by supplier

My users want to know, for a given supplier, how much we owe them in unpaid invoices. So where the invoice has been created, but not yet paid.

I have created a report for them and now we need to verify that the report is correct.

There is a liability account that contains this amount, but it is not broken out by supplier. Also the user says that this account and my report would only agree immediately after the month-end close.