Hi Anjli,
In the standard, you cannot enter an internal order number (COBL-AUFNR) on header level, only at item level. VBAK-AUFNR at header level is used for service processes (resource-related billing) and it has nothing to do with the account assignment at item level.
The field selection logic for the account assignment screen of the sales order is hard-coded in the following programs:
FV45PF0C_COBL_SEND_PBO_VBAK (header level)
FV45PF0C_COBL_SEND_PBO_VBAP (item level)
You can use the following user-exits in order to change the standard field selection logic:
USEREXIT_COBL_SEND_HEADER (MV45AFZB)
USEREXIT_COBL_SEND_ITEM (MV45AFZB)
Best regards,
Crystal Lin