In early R12 releases, the ability to generate negative usage invoices was removed (except for settlement) and the profile option OKS: Enable Negative Invoicing was disabled; zero value billing data was generated instead of a negative invoice.
The ability to bill negative quantities for usage is now restored in R12.1.3+ with Patch 18233437:R12.OKS.B. Please see Doc ID 1665321.1 for details.