Have you tried to explore Approval Flow configuration? If not please take a look at it. https://help.sap.com/docs/buying-invoicing/invoicing-and-payment-process-guide/services-invoicing-workflow-in-sap-ariba-solutions-b56fab0c7bed49128c0f2d01cca6a363...
You can use any middleware which can invoke WebServices using SOAP format. There are webservices available in Sourcing and you can integrate with any system.
