Line Type/Inv. Interface with Item Information Except Cost

Hello,

Can you please suggest which Inventory Interface is required if the Item information needs to be brought to the Purchase order except for the Cost?

We have a custom program that writes data from another table into F4311, so I setup the Inv. Interface = 'B' so we can default the G/L Category Code from the Item information, but the standard BSFN in P4310 also updates the Cost, even if we are passing this value (this is a Service Purchase Order- Non-Stock, but we use Stock Inventory Items).

Thank you.