A company may deal with a supplier over many years, and over that time it will occasionally return products or protest incorrect billings that result in credits being granted by the supplier. The trouble is that the supplier may not send formal notice of the credit to the company, so the accounts payable staff has no idea that there is a credit available that it can use to offset other billings issued by the supplier.
The solution is to request that a formal statement of outstanding invoices and credits be sent to the company by every major supplier at least once a year. These statements should identify all open credits, for which the accounts payable staff can either request repayment or use as offsets to new billings. The key elements in this best practice are (1) to monitor which suppliers refuse to send a statement, so that follow-up action can be taken, and (2) proper review of the statements by the accounts payable staff, which may not want to spend the extra time on this chore.