Solution : https://service.sap.com/sap/support/notes/1640940 (SAP Service marketplace login required)
Key words : 
report showing normal customers, symptom report 'saf-, xml file creation, user-exit-43 reason, relevant support package, attached correction instruction, working correctly, negative invoices, 'taxpayable' field, time supplier
Related Notes : 
       
| 1459306 | SAFT-PT: Collective note for SAFT-PT notes |