How do I disassociate a site from a domain name?

I have multiple sites for our business that I am currently working on. I finished a site and registered for a domain name, which was rejected after the registration process was finished.  So I registered for a variation on the rejected domain name and successfully registered the new  domain.  But the new domain name associated with the wrong site that is still under construction and when I tried to change the association of the domain over to the correct site I get an error message that says I must disassociate the correct site with it's domain name to change the association of the new site name over to it.  Problem is that I can not find where the site is associated with a domain name at all or where to disassociate sites from their assigned domain.