How to configure postfix to permit relaying from some ip address but only using one domain as sender

Have a look at: http://www.kutukupret.com/2009/12/28/postfix-selective-sender-addresss-verification/

The idea is to create an smtpd restriction class which does not verify the sender address and use the access map so that only the domain you don't want to check matches that class.

main.cf
#relayhost = [smtp.domain.net]:587
transport_maps = hash:/etc/postfix/transport

/etc/postfix/transport
domaintodeliverdirectyto :
* smtp:[smtp.domain.net]:587

Don't forget to postmap /etc/postfix/transport

May want to use relay instead of smtp as the transport mechanisim.
For reference http://www.postfix.org/transport.5.html