I think Michael's answer is an excellent one. My only reservation about his suggestions is that, in my experience, if your opponent puts up any kind of defense to your civil claim and it extends for any length of time, the attorney's fees could easily and quickly go to a level that sucks the guts out of the $36,000 you hope to get back. I second Michael's criminal restitution suggestion if for no other reason than that the fees will be much lower.
You have three basic options. The first is to continue to work informally with the person or company to try to get them to voluntarily repay you. The second is to take the case to the police or attorney general and try to get the state to press some form of criminal charges. If eventually convicted, either by plea agreement or trial, a restitution order can be entered requiring the defendant to repay you, though collecting on restitution orders is often a slow process. Finally, if you are interested in things like garnishing wages as you mention, you will have to file a lawsuit against the defrauding person or company. Filing a civil lawsuit is the only way that you can get a judgment, and you need a judgment before you can start any kind of collection action, such as garnishment of wages.
The written admission that you have will likely assist you in moving the lawsuit forward to a judgment, but don't think of it as a guarantee. When faced with a lawsuit, the other party may well change their tune, deny that the writing is legitimate, claim it was written under duress, etc, all of which will force you to litigate the matter more before you can get a judgment. Of course, you also need to keep in mind that getting a judgment does not guarantee that you will be paid, as the other party might file bankruptcy or might simply have no assets and limited income, making them "judgment proof."
In any event, this kind of case can be fairly complex, and the amount of money at stake would appear to justify hiring an attorney to help you with this case. There are likely others in your same situation, and with what is likely an unsecured debt to you, the sooner you get a judgment, the higher your priority in the list of creditors against this other party.
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