Appraisers

I have a listing were the appraiser went out did the appraisal, told the buyer that the seller bought the house for less than what it was being sold for, which didn't bother me as much as SAYING the house was a MOLD house. Which it never was. I confronted her about it, and she said the previous listing agent told her that. Asked the previous listing agent, and of course she never siad that. The buyer 2 days later backed out saying there was undisclosed mold. We have pics in the MLS showing no mold anywhere, pre rehab photos showing the same. Is there anything that can be done about something like this, or has anyone encountered this before. Thanks Jeremy