Hi Bhargava,
If they want to send PO data as an XML file, that should be fine. Ask them to write the file in common SAP landing zone (accessible through AL11 transaction), so that PI can pick up the file through NFS protocol of File Adapter and read (as the file is already in XML format you don't need File Content Conversion in Sender File Communication Channel). After that perform mapping and all required steps including configuration. So PI will read the file, perform transformation to SOAP message and send to the web service.
Regards,
Nabendu.