Rahul,
Create a SOAP to Proxy (or IDoc) scenario in SAP PI. Sender SOAP channel will be async (exactly once).
If source system can send XML, define that as source structure.
If source system want to send a text attachment, define dummy source structure. And use payload swap bean in sender channel.
FYI, You can test this scenario, using WSDL from sender agreement using SOAPUI.