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In criminal law you can accuse them or fraud and attempting to obtain money by deception since they are endeavouring to obtain the purchase price back by lying and by using threats of libel/ slander against you knowing that a bad review may lose you business they are also trying to blackmail you.
The fact they have done this previously would work against them both in a criminal prosecution or a civil law case as it shows a pattern of behaviour.
In civil law you would sue for libel and/ or slander. Libel being the written word and slander the spoken word. You can sue for any financial loss they cause you as a consequence of lying.
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