In a typical sale, you have the buyer and seller (both of which could be multiple people but still a two party agreement). It sounds to me like this is probably a short sale. if you're unaware of how a short sale works, it is when a seller needs or wants to sell their home and they owe more to the bank than their home is worth in the current marketplace. In those instances, the bank is the third party and has to approve the sale before the contract is considering fully executed.
Century 21 Princeton Properties