OASIS supports both credits and negative invoices. The difference between the two is based upon how they affect the order.
Credits in OASIS reflect that the customer has been credited a certain amount but DOES NOT put money "back" onto the order increasing the amount left open to invoice. Credits appear on the order and show that the customer has been credited but do not cancel out the invoiced amount so the order will also show that the customer has been invoiced. In the example below the entire amount had been invoiced and refunded through a credit and both are reflected in the customer and vendor invoice fields.
Negative invoices "put money back on the order." In other words, the open amount on the order will increased by that amount and it will be as if the original invoice never happened. This works best in circumstances where the wrong items were shipped and the original items will need to be monitored and reinvoiced at a later date. As you can see in the example below, even though there is an invoice and a negative invoice shown for the customer, no values are reflected in the customer invoice and customer credit fields.