You are not able to set Overall Discount in the Over-The-Counter (OTC) web service interface. While, it is possible in the UI.
SAP Business ByDesign
Reproducing the Issue
Overall discount is not supported for OTC via web service. Header level condition Overall discounts are not accepted in the request.
This is the currect system design.
In Sales Order, overall discount is supported. Find more in the blog https://blogs.sap.com/2018/11/25/discounts-in-sales-orders-and-contracts-sap-business-bydesign/.
web service, OTC; overall discount , KBA , SRD-CRM-INV , Customer Invoicing , Problem