When you receive a credit for freight charged from a vendor on an already finished PO, the best approach is to add a negative expense PO (expense with a minus against the dollar values), and link it back to the original PO (add the PO# in the GL grid), so it reduces the cost of the goods on the original PO.
This will reduce the cost of goods, and add a credit to the creditor's account.
You will see some letters underlined in the field names within the header, eg. Status. Using Alt+ the underlined letter will jump you to the field beside that heading.