Solution : https://service.sap.com/sap/support/notes/378726 (SAP Service marketplace login required)
Key words : 
additional key words, attached source code, payment clearing items, payment items, payment clearing, payment requests, payment request, payment item, sales order, final invoice
Related Notes : 
       
| 528608 | Sales document with 'pure' down payment items (new) | 
| 306151 | Sales document with "pure" down payment items (old) |