I copy trading contracts (SAP GTM) using function module BAPI_TRADINGCONTRACT_CREATE. First I use function module BAPI_TRADINGCONTRACT_CHANGE to read the contract I want to copy and then I create the new trading contract with function module BAPI_TRADINGCONTRACT_CREATE.
On the trading contract I have sales conditions marked as delivery costs (KNTYP = B). For these conditions the vendor field is displayed when double-clicking on the condition.
The problem is when creating trading contract with BAPI_TRADINGCONTRACT_CREATE I can't update the value in the vendor field.