If you simply need to save off a copy of what get’s sent, then the suggestions from ramesh and Mark will do nicely. Be aware that you’ll need to pay attention to disk space if you simply write it to local disk, depending on your message volumes and the size of your server’s disk.
If you want a bit more capability, you can use Partner Manager.
It’s been a while since I’ve worked with BC so I may be mixing things from that and the wM SAP Adapter but I believe the overlap is significant.
Under the covers, the BC components use a package named WmPartners. This is the tool that provides the routing based on routing rules you set up. In the service the does your IDoc translation, instead of doing the HTTP post directly, pass the XML string to the appropriate service in the WmPartners package. There is documentation for this in the <integration>/packages/WmPartners/pub/doc directory.
Set up another rule for the right sender & receiver & this document type. Configure the rule to invoke a new service that you write (containing the parts of your existing service that does the http post) to send the document to the partner.
By following this approach you get all the facilities of WmPartners for your XML document (saving to the message store, dynamic routing, etc.).