Buyer threatning to sue me

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Hello. Have a house for sale and had buyer #1 make a full cash offer, close in 2 weeks. I said great! But when 1st closing came they didnt have all the money and their agent kept on giving is run around about POF. So we extended closing date 7 days, which in that time frame I jad another very interested cash offer buyer with POF. Now Buyer #1 is saying they are getting a private lender involved because they dont have all the funds! Second closing date came and wanted another extension so i declined. I accepted buyer # 2 offer, now Buyer #1 attorney wants to threaten me that he is going to sue, they are not signing mutual agreement and now im confused since its my first sale, On what grounds will they sue? I have no idea, since they were in breach by not closing on time. Should I be worried? Or is this a scare tactic to make me sell my home to them?