Vendor No# is not in Company Code, but SAP allow to create PO ?

Question

Hi All,
In the Purchasing Order (PO) header, we create Purchasing Org, Purchasing Group and Company Code "A" .
However, users input incorrect Company Code "B" in the PO and the vendor do not exist in Company Code "B" . PO created is successfully, but error out when user input invoice against the PO.
Anybody know how to prevent user input incorrect company code in PO when vendor do not exist in the company ?
Thank you
Alex

Answer 1

from FAQ note 491789
Why can I still change the company code  at header level of the purchase order although follow-on documents (goods receipt, invoice receipt ...) already exist?
Answer:
In the purchase order there is a company code at header level (field EKKO-BUKRS) and a company code at item level (field EKPO-BUKRS). The company code at item level is relevant for the update of the FI and CO documents. This company code is always derived from the plant and therefore you can no longer change it.
What significance does the company code have at header level of the purchase order?
Answer:
The local currency and the corresponding conversion factors are determined from the company code on header level, as well as the VAT registration number (VAT reg.no.).
for PO creation the vendor does not need to have the  company code data views maintained. This speeds up the process, you can already order while Accounting has not finished the data maintenance. CC data is first time  needed when you like to process goods receipt and invoice receipt.