Invoice line won't accept negative number for unit price: how to do return credits?

Oracle Utilities Work and Asset Management (WAM) V2.3.0.0.0 134 000

Oracle Utilities Application Framework (OUAF) V4.4.0.3.0 007 000

I'm attempting to post a line item for a return credit invoice. We're attempting to switch from WAM 1.9 to WAM 2.3. In 1.9, we could simply INSERT a negative value into SA_INVOICE_ITEM.INVOICE_ITEM_AMOUNT. However, when we attempt to post a negative value to W1_INVOICE_LINE.UNIT_PRICE (via the OUAF API with a CM-InvoiceLine BO), we get the error "Item amount must be a positive number."

Please note that these invoices don't have a PO associated with them. Even if they do, it doesn't look like we can enter negative values into the item amounts.