Hi Sabine,
yes, you have to specify all the values for all the fields, that should be filled. The omitted fields will be cleared (in the Header, Addresses...).
The outbound type doesn't matter if you process the update with a "call B1" - atom. I've personally used Void in my test scenario.
The error occurs, because the tag <PriceListNum> is missing in your Payload. Unless you'd like to change this value, leave the current Price List of the Business Partner being updated.
Best regards
Alex.
