Solution : https://service.sap.com/sap/support/notes/990271 (SAP Service marketplace login required)
Key words : 
budget billing request, fkk_create_doc_from_logical function module, partially cleared, full amount, tax amount, written correctly, reumws00 report, sales/purchases, vat reason, program error
Related Notes : 
       
| 987156 | REUMWS00: Various changes in the program | 
| 985097 | ABP: Tax determination with due date not possible | 
| 984842 | Change to sales/purchases tax rates in Germany Jan 01, 2007 | 
| 971906 | Budget billing plan: Tax change | 
| 966902 | REUMWS00: No recalculation if partial clearing is reversed | 
| 965447 | REUMWS00: Additional information in the protocol | 
| 943768 | REUMWS00: Taxes on sales/purchases is not recalculated |