Here's the situation:

We received a customer PO and processed it under a new sales order.
Sales Order was confirmed.
The shipment was staged, shipped and the delivery was validated.
Invoice was generated and validated

Next day, Inventory clerk reports that a wrong product was shipped.

How do we fix this in Odoo?

Here's what we have figured, so far:

Delivery of incorrect product was returned & validated.
Reduced quantity of wrong product to zero, in Sales Order.
Added actual product shipped.
Validated delivery

I now have an invoice for -3,200 products. When I add a price to the line item, I get this error.
"You cannot validate an invoice with a negative total amount. You should create a credit note instead."

The tool bar does not show the option to "create a credit note", as it would with a normal invoice.

Please, assist however possible.