Whether you're at fault or not, the money wasn't yours and therefore you must return it to its rightful owner. If the return preparer makes its client whole by paying the client for that money, then the preparer will be able to collect that money from you if you do not repay it.
Keep in mind that there may be criminal consequences as well as civil consequences if you don't immediately contact the people involved and try to work out a way for you to repay the money.
If you cannot repay the money in full, then I would strongly advise you to speak with an attorney who has criminal law experience and to not say anything more about this matter in public or online.
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Even though this happened to you "accidentally", this is the kind of situation that can have difficult and even criminal repercussions, particularly since you already spent the money and cannot repay it. You need to have a confidential discussion with a tax attorney about the possible options and consequences of actions you may take to try to resolve this matter.
This information is provided for educational purposes only, and is not to be relied upon as legal advice. You should consult with an attorney with full disclosure of all facts and opportunity to consider all or alternative options.
I agree with my colleagues. You have to reimburse the person to whom it rightly belongs, or reimburse the preparer, who's insurance likely already paid a claim. Hire an attorney right away or you could be facing serious trouble.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/
You are surely liable. Once the individual (that did not receive their return) files a police report, it will open you up to liability right away. Coming forward on your own would be the best thing. Start saving up some money.
You will definitely have to repay it. It is likely that a payment plan can be worked out. This is one of those cases where the longer you take to respond in good faith willingness to repay (even if you lack the current means to repay), the more likely it is that criminal charges or other penalties will make this situation much more difficult.