I'm afraid that the XML logging feature of WinSCP is completely irrelevant to your task.
Other than that I'm not sure, if I understand your assignment.
It seems to me that all you need is to download an XML file from the server. For this, see the guide to automating file transfers from SFTP server.
If I'm wrong, please try to explain, what do you mean by "service(B) will send XML back to service(A)".