Hi ,
I am not sure how u can do will you consume the web service and send data to NAV as I am not experienced in DoTNET.
From NAV what you need to do is -
1) Create a codeunit with functions as per your requirement.
Parameters as Item No & Location Code
Return Value as Stock as that position.
2) Publish that web service.
3) Consume that web service (may be some one else can help you on that part).