Foreign Trade : Default Business Transaction Type not set for Return STO

Hi My scenario is:

I have created a Return STO  (Inter company PO with Document type NB) - and trying to create subsequent documents.

While I am creating the Billing Document I get an error : Foreign Trade Data Incomplete.

I have done the required setting  : i.e. maintaining the default value as per following

Foreign Trade --> Basic Data  for Foreign Trade-->Define Business Transaction Type and default value-->Default Business Transaction Type (MM)

Still when I create the Billing Document the default value does not get picked up and we get the error of foreign data incomplete. Can you please help?

Regards

Rahul Soman