My recommendation is to start from scratch and advise the real estate company to hire a lawyer for breech of contract or harm to their business as this is causing them economic harm.
We were in a similar issue once when we had to drop a client, yet we still owned the copyright to the site's design since we had purchased a template for their site and it was non-transferrable to the client or their new company and our former client knew that and it was in the contract.
We were still a brand new company, only about 2 months old, but what they didn't know was the two primary investors had a combined total of about USD 600 Million dollars in cash. Needless to say we sued both parties out of business (one for breech of contract, the other infrinigement) and purchased their assests at liquidation prices just as an *#$&# you.
Funny thing is, we haven't had any problems since...