If I was guessing (without fully understanding the syntax!) Id say the problem was
 
Billing-Party=$avp(billing_party);
 
should be
 
Billing-Party=$avp(s:billing_party);
 
Although in theory both of the above config entries seem to be valid, though different, openser.cfg entries. According to this:
 
http://www.openser.org/docs/modules/1.2.x/avpops.html
 
:S
 
Also, I tried with the second format (with s:) and that then gives me another error when I start openser:
 
 0(0) INFO:xl_parse_name: using hdr type (27) instead of <user-agent>
 0(0) ERROR:acc:parse_acc_extra: parse failed in <User-Name=$Au; Calling-Station-Id=$from; Called-Station-Id=$to; Sip-Translated-Request-URI=$ruri; Sip-RPid=$avp(s:rpid); Source-IP=$si; Source-Port=$sp; Canonical-URI=$avp(s:can_uri); Billing-Party=$avp(s:billing_party); Divert-Reason=$avp(s:divert_reason); X-RTP-Stat=$hdr(X-RTP-Stat); User-Agent=$hdr(user-agent); \
Contact=$hdr(contact); Event=$hdr(event); SIP-Proxy-IP=$avp(s:sip_proxy_ip); ENUM-TLD=$avp(s:enum_tld)> around position 319
 0(0) acc:parse_acc_extra: error
 0(0) ERROR:acc:mod_init: failed to parse rad_extra param
 0(0) init_mod(): Error while initializing module acc
ERROR: error while initializing modules