An Invoice Reconciliation (IR) document has an Invoice Unmatched exception, even though the Purchase Order (PO) is provided correctly on the associated invoice document.

ANID is maintained on the wrong supplier or supplier location record in the SAP Ariba Invoicing solution.

First, ensure that the supplier and supplier location match on the PO and invoice.
You can check the invoice cXML directly in your buyer SAP Business Network account, and check the <From> credentials in the invoice and ensure they match the <To> credentials on the PO.
Specifically, the vendor ID and the SAP Business Network ID (ANID), if present, must match.

If they do not, and the invoice is an ICS invoice, reach out to your supplier to ensure that they send the correct data.

If the invoice is an online invoice, ensure the following:

• If you have multiple suppliers in the SAP Ariba Invoicing solution that are related to the same SAP Business Network supplier, ensure that you only maintain the ANID for the supplier in your buyer SAP Business Network account.
  Do not maintain the ANID on the supplier records in the SAP Ariba Invoicing solution.
• If you have multiple supplier locations for a supplier, do not maintain the ANID on any supplier location in the SAP Ariba Invoicing solution.

Invoicing > Invoice Reconciliation