Issue a full credit note cancelling the given partner invoice. The credit note copies the original's seller, buyer, currency and payment terms, negates every line item, is marked paid, and references the cancelled invoice via referenceId. Only invoices generated through the built-in B2B invoice integration are eligible, and only one credit note may exist per invoice. Issuing returns the linked settlement report to ready_to_invoice. Partial credit notes are not supported.

A 409 means the invoice is permanently ineligible and the request will not succeed on retry. It is returned when the invoice already has a credit note, when the target is itself a credit note, or when the invoice was not generated through the built-in B2B invoice integration.