https://userapps.support.sap.com/sap/support/knowledge/en/2837404
2837404 - FKKBIX_BILLREQ: Quantity and Unit of Measure | SAP Knowledge Base Article
In transaction FKKBIX_BILLREQ or FKKBIX_BILLREQ_MON it is not possible to change the field quantity (MENGE) in the billing request for correction (credit memo...