All appeals are dismissed.

The costs of appeal between Plaintiff C and the Defendants are assessed against the Defendants.

1. As to the plaintiffs' grounds of appeal (to the extent of supplement in case of supplemental appellate briefs not timely filed)

A. According to the instant waiver agreement, the lower court, even if the contract deposit is concluded prior to obtaining permission from the competent Do Governor for the land within the regulatory zone under the Act on the Utilization and Management of the National Territory, where one of the parties fails to carry out cooperation for obtaining land transaction permission or withdraws the contract before filing an application for permission, may have the agreement to compensate the other party for a certain amount of damages valid.

(2) In light of the above legal principles, the Plaintiff Co., Ltd. and the Plaintiff B (hereinafter “Plaintiffs”) concluded a sales contract on the instant land owned by the Defendants located within the land transaction permission zone and concluded a new construction contract on the instant land, but failed to pay the remainder on the initial payment date and the deferred payment date, the Plaintiff Co., Ltd. and the Plaintiff B (hereinafter “Plaintiffs”) concluded a contract to waive all rights related to the instant contract, and concluded a contract to give up all rights if the remainder payment date is not made within the extended payment date, and concluded a contract to give up all rights related to the instant contract. The instant sales contract was in the state of flexible invalidation at the time of the instant contract. However, even if the Plaintiff’s buyer promised to pay the remainder until the extended payment date and fails to perform the contract, it is possible to conclude an agreement to pay the remainder as the down payment amount separately from the instant sales contract, and according to evidence, the remaining amount is not paid until the extended payment date by the Plaintiff’s buyer, and thus, the Defendants were deemed liable to compensate the Defendants for damages.