Solution : https://service.sap.com/sap/support/notes/495805 (SAP Service marketplace login required)
Key words : 
set item attribute process_qty_unit, add item attribute process_qty_unit, sap internet sales reason, access attribute process_qty_unit, variable key field, vkp0 condition type, vkp0 condition record, condition table, sales prices, sales unit
Related Notes : 
       
| 413617 | IPC Installation 3.0 |