An agreement of sale is a contract, therefore it is best to consult a lawyer before termination to determine what liability you may have. Has the seller accepted the responsiblity for cleaning the mold? If so, and they can prove that the mold has been remediated, he or she has fulfilled that responsibility. Have all the other contingencies of the contract been met? Depending upon how the contract was completed, you may only stand to lose your hand money. I do hope that you are working with a buyer's agent who will guide you through this critical time in the contract.