first thing you can do is document what a "valid request" looks like at the raw HTTP level.
ie grab the incoming text in the server log and paste that into your docs. (be sure to obfuscate the bank account details.)
second document all the possible responses at the HTTP level. (you can add a project define to your server,
NetShowSend=>1
and then the responses will appear in debugview.
In my SOAP Webinar I used a weather service as an example, and they had really nice documentation, so creating your documentation to look like theirs is a really good place to start. That does the most good to the most number of languages.
Of course, being a .Net developer, he's gonna look at you confused like, and ask for a WSDL file. This is a text file containing a description of your service. .Net creates, and consumes these files automatically, which is nice, but results in programmers not knowing how to read the raw HTTP. So most (although not all) .Net programmers I've come across are lost without this file.
It is possible to create a WSDL by hand - typically take a basic one and modify it to match your interface. A bit of trial and error with him perhaps, but it's not especially difficult for simple interfaces like yours.
Cheers
Bruce