How to pass the client's IP address through a VPN connection

The short version of this question is: Is there a way to set up a VPN firewall rule that does not NAT the clients IP address and instead just passes the client assigned IP address through?

I have two applications sitting behind an ISA 2006 server that are apparently not returning the requested information back to the ISA Server in a way that it, the ISA Server, understands and can then send back to the connected VPN computer.  One application is called Munis and is a municipal accounting program.  I can login to and get to a terminal session that allows me select where I want to go from there.  It displays the address it believes me to be at.  As an example, I obtain 10.90.71.25 as my VPN client IP address but Munis displays the address of the ISA Server.  When I select the Munis module I want it thinks for a minute then dies.

This is my theory and you have a better one I'm game to hear it.  I am new to ISA Server and welcome any suggestions.

Thanks,

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