The contract is binding on your husband but not binding on your father in law.  This means that if the broker finds a buyer, your husband must pay the fee.  The father in law won't pay the fee.  Talk to the broker and tell her that your father in law won't sell the house through a broker.  He wants to sell to a private party and ask the broker to cancel the contract.  The property can't be sold without the signature of the FIL.