You are creating a supplier invoice (without a reference of purchase order) via a web service.
While using the web service request, you are unable to update the Deductions And Surcharges tab under item level.
This is the normal system behavior as Business ByDesign does not support the creation of deductions and surcharges for supplier invoices via web service.
You could add this requirement in our forum: https://influence.sap.com/SAPByDesign.
Supplier Invoices, web service, deductions and surcharges. , KBA , AP-SIP-SIV , Supplier Invoice , How To