Help in ISA 2004

We are accessing a remote client through a VPN and when we connect to that client without the ISA the client software works perfect.

but when we route the connection through the ISA connection get dropped. (ISA 2004 SE)

What happen is as below;

from our internal network Request is generated using port 603 and when it directly goes to the client end client get it on port 513 (rlogin)
but when we route the connection through the ISA request generates from Port 603 get changed to 2XXXX range when it goes out from our router and client drops the connection (Rlogin requires the port to be lesser than 10XX)

any advice greatly appreciated.