Thank you for your reply, although I don't see where you addressed the issues of whether or not you have a contract that covers this issue and also whether there is some specific step that your employer is taking that you have a question about. From the information that you provided, it appears that you already signed a promissory note, which is an admission that, at least in part, you owe the employer money. At this point, you can't unring that bell. If you had not already signed the promissory note, I would suggest that you consider refraining from making an admission of owing any debt, which includes signing a promissory note, until the employer provided you with all of their evidence of the debt.
The employer may choose to perform an audit, at this point they don't have to, especially if you signed that promissory note. Certainly if they were to file suit or refer this as a criminal complaint, you would ultimately be able to force them to produce proof. However, from a job retention standpoint, they can terminate you without any proof of misconduct if you are employed "at will" which you would be unless you are in a union or have a written contract that states otherwise.
If you have specific questions, I would be happy to answer them, but at this point, without knowing exactly what specific information you are looking for, the information I have provided covers what I see as issues thus far.
Again, please feel free to ask follow up questions. If none, then if you could leave a positive rating of my efforts to address the issues you have raised, in the ratings box on this page, the the Site will give me credit for assisting you today. Thank you