Charlie,
as you can see in your config, you have 'no nat client'.
This is the source nat.
All you have to do is create a pool of address and then assign it to the serverfarm with 'nat client '.
This will nat all traffic from any client.
If you want to avoid this, you need to create a 2nd serverfarm that would be used exclusively when connection is made from server A.
Let me know if you need anything else.
Regards,
Gilles.