Port forwarding to internal proxy

[merlin81]

Hello,
I've a proxy in internal LAN that should be used as transparent proxy from pfSense 2.3.
pfSense has a simple setup, using one WAN(DHCP) and one LAN (192.168.0.1/24) port:

WAN      LAN
  \            /
  pfSense

The proxy (192.168.0.10) is setup to listen at port 3128 and all http/https traffic has to go over it excluded traffic from 192.168.0.10.
I've tried several variants of NAT settings on LAN interface, but nothing as expected. Setting up NAT for one source address (192.168.0.20) stops all traffic http/https traffic from this host to the internet, a tcpdump at the proxy does not show any package going to the proxy.

Can anyone give me an explanation or example how to setup forwarding to the internal proxy ?

Thanks in advance,
Christoph