Dietrich,
by default the CSS will keep the original client ip address.
To have the CSS changing the client ip, your customer must have configured a group with 'add destination service'.
Probably because your client is using a one-armed setup which is the easiest to implement but the worst to use.
So, your customer should go to a 2-sides CSS design and have the traffic flow through the CSS without the need to do client nat.
Once the design is correct, you can remove the group and the CSS will keep the client ip address.
Regards,
Gilles.
Thanks for rating.