Solution : https://service.sap.com/sap/support/notes/1043419 (SAP Service marketplace login required)
Key words : 
'daily simple interest method', attached correction instructions, leasing transactions, amount financed, 3 monthly installments, 900 eur independent, period length, monthly installments, terms ff13a, saplff13a reason
Related Notes : 
       
| 1060663 | New FiMa: Incorrect installment determination for loans | 
| 1055250 | New FiMa: Incorrect installment determination for loans | 
| 1049968 | New FiMA: Period-related installment determination |