Negligence on the part of the new mortgage holder who is required to pay real estate taxes is not your problem. Write them a letter disclaiming responsibility and that they are to pay such additional costs. If they refute same then hire a real estate lawyer to write a letter referring to the negligence and the specific terms of the mortgage instrument. Is not your fault....stand fast on this. If you cannot afford a lawyer if they insist on making you pay then seek out the attorney general in your office as they need to know about financial institutions abusing a consumer....lots of laws abound to protect you on stuff like this, good luck to you.
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