IP source rewriting with fortigate

Hello, I have a fortigate on which I have a VPN that works well. My concern is that a subnet is repeated on both sides, a specific machine on one side must communicate with others on the other side but as his address is repeated these requests are redirected elsewhere. I would like to do a NAT by giving a public IP address to reach and arrived at the firewall I assign a NAT. except that for the return I have to change the source address, how could I do this on a fortigate 200D?