The supplier needs checking to see what postings have been done that could of affected the balance, this can be done via the Purchase Enquiry screen:
From the main menu select Purchase Enquiry.
Put in the supplier you require either by searching for the name with the filter in the top header bar or using the number if you know it.
Highlight the supplier and then select the Postings tab at the top.
All the postings that have effected the supplier will be on there and you can check the list to see if any postings have been done that didn't go through a matter.
You can add a search at the top of the screen for references etc, the postings can also be drilled down into by double clicking on the posting.