Solution : https://service.sap.com/sap/support/notes/531754 (SAP Service marketplace login required)
Key words : 
import relevant subcontracting purchase order, terms foreign trade, condition type defined, condition gwlb, lb-component, calculated correctly, price unit, prerequisites calculation, advance correction, subcontracting
Related Notes : 
       
| 747665 | Calculation of GWLB condition, while posting GR for Sch.Agr | 
| 446833 | ME21, ME22, ME21N, ME22N: wrong price in subcontracting item | 
| 397846 | Import relevant SC order, GWLB not determined |