Hi Mikael
Welcome to SCN!
If you are always sending the same XML payload to the target SAP system, I guess that it should be populated with all constant/hardcoded values.
If that is the case, you don't need an actual source XML payload. Just define a dummy structure for the source message type. In the message mapping, just map Constants to all the required fields in the target payload. This way, it doesn't matter that there is no XML payload sent to PO when the service is called.
This is somewhat similar to a scenario mentioned in my blog below, except that it is an empty response instead of an empty request. Refer to "Empty Response body" in section "Additional points for consideration".
Consuming RESTful CRUD services with Advantco REST adapter
Rgds
Eng Swee