Solution : https://service.sap.com/sap/support/notes/827487 (SAP Service marketplace login required)
Key words :
execute transaction se80 select function group, goto field gt_screen_itm-partner_prod change, enter bbp_cf goto screens, vendor product number, terms bbpcf02 reason, element list tab, change mode, backend po, delivery flag, program error
Related Notes :
| 892206 | SRM invoice: Enlarge Vendor product number to 35 chars |
| 887581 | SRM invoice: Vendor product number not transferred |