I'm not sure if "somebody" refers to someone you did business with, such as a contractor, or fraud from an unknown party. Assuming this was a mechanic's lien (a type of lien filed by a contractor) and the lien was not properly filed and notice was not properly given, you can sue the contractor for filing a spurious lien. For more information see our article Fight the Lien on Your Home on what a mechanic lien is and what your options are.
Even if the lien was completely fraudulent and have nothing to do with a contractor, the following options would still generally apply. (If you are the victim of a crime you may have more options available to you.)
Pay the lien. Obviously, you'll be out $13,000, but the sale will go through.
Contact the filer to attempt to negotiate a resolution. This is the least expensive option, but you will have to deal with it. Of course, if the lien is completely fraudulent then this option would not work.
Allow the sale to fall through and then sue the contractor/filer. Assuming the lien is spurious and you prevail, you have the potential to recover the most amount of damages from the contractor/filer and will not have to pay off the lien. Downside is the sale will fall through, and you take on the litigation and collection risk. You could also have liability to the buyer, which would largely depend on the terms of the purchase and sale agreement.
Have an attorney contact the contractor and send a demand letter to immediately release the lien before you suffer additional damages. Possibly the fastest, but would require that the filer engage and not ignore the attorney.
File a surety bond with the court, which allows the lien to be released and the sale to proceed. This option allows your sale to proceed but also preserves your right to sue the contractor/filer. This option is likely the best given your scenario, however, it will likely take the help of an attorney to accomplish it. It would cost more than a demand letter, but less than a full lawsuit.
Because I only have a few facts, everything should be treated as general information. It is important that you speak with an attorney as soon as possible. If you would like to speak to an attorney at R&H, feel free to contact us at (303) 688-0944 for a free case assessment.