Sounds like you did it backwards, Addie.
I don't think the sign shop needs your blessings to use the copyrighted stuff, but your customer does. Seems as if you might've given the wrong permission to the wrong people and now you're in a fix.
As Forge asked.... do you have a signed contract or just some back & forth e-mails ??
Are we talking about a franchise real estate company or some 'One-Man' office with several agents ??
Unless you have a signed agreement with your real estate customer that they turned you down as their authorized sign shop, but kept the logo and don't have any documentation that you offered it to them at the time of sale to them.... you can't go back now, after you've given it out for use. Regardless of how you word it, it's going to be hard to prove this ain't 'spilt milk' and you're just upset.
I don't know how long ago you actually got paid for delivering a logo to this company, but unless you have signed paperwork of their turning you down on an offer for them to purchase the rights to the logo, in my opinion, you don't have a leg to stand on. You being the professional here, the designer and the initial start.... if you didn't explain to them upfront of all the "in's n' out's" of the legal mumbo/jumbo, I wouldn't find in your favor on anything here. You can't hold a company or business hostage if you never told them of the ramifications if they don't use you and still use something they paid for thinking it was a 100% finalization type of deal.
When someone comes to us and asks for certain artwork/logo/design to be created.... all of the information would be outlined and explained in writing so they can sign it before you start anything. All parties involved must know the beginning and the ending before entering into a contract; otherwise, you're guilty of not fulfilling your obligation as a contractor. I’m sure it’s a little different down there in Florida, but business is the same everywhere. It’s not good to deal with half facts and then come on some forum and get information on this stuff and try to recoup on old accounts with force. Have you even tried contacting the real estate guy and asked for some money out of pity for your lack of contract etiquette ??
Another good one….. a lawyer on contingency ?? You'd be better off defending your own bad actions and taking your chances. A good lawyer will probably tell you to forget it and start doing things by the book.