2. The plaintiffs' assertion and judgment

A. The plaintiffs asserted as follows: ① the defendant should cancel the registration of creation of a mortgage on the instant building immediately after he received the down payment from the plaintiffs pursuant to the instant special agreement, but the above building was sold to a third party without implementing it; thus, the instant sales contract was lawfully rescinded, or ② the defendant, at the time of the conclusion of the said sales contract, had the intention or ability to cancel the registration of creation of a mortgage on the said building immediately after he received the down payment from the plaintiffs, by deceiving the plaintiffs, and received the down payment, so the above sales contract was lawfully cancelled by the delivery of a preparatory document as of August 21, 2014, and thus, the defendant is obligated to pay the plaintiffs the restitution and compensation for damages incurred therefrom.

B. Even if the seller’s obligation to cancel the registration of the establishment of a sales contract of this case was more preferentially performed than the buyer’s obligation to pay the remainder, the seller and the buyer did not provide the buyer’s obligation to pay the remainder, and as long as the due date for payment of the remainder is too excessive, both parties’ obligation are placed in a concurrent performance relationship. In a bilateral contract in a concurrent performance relationship, a person who wishes to cancel the contract on the ground of the other party’s non-performance of obligation must provide the buyer with the performance of his own obligation