Hi there,
I was wondering as well, so I took a brief dive into the code.
Apparently, billing was disabled somewhere along the way. In fengoffice/application/views/account/system_permissions.php, columns 'can_view_billing' and 'can_manage_billing' are hidden from the permissions administration interface. This also concurs with something I read (can't remember if it was in the forum or some place else), saying that billing would be disabled until it worked...
There must be something else though, because simply un-hiding these columns and granting these permissions to a contact is not enough to allow that contact to really manage billing. The only one who can is the Super Administrator.
If I ever find something else in these regard, I'll try and report back here.
Cheers
--
Romain