The appeal is dismissed.

The costs of appeal are assessed against the Defendant (Counterclaim Plaintiff).

The grounds of appeal are examined (to the extent of supplement in case of supplemental appellate briefs not timely filed).

If the performance of an obligation was not possible at the time of the contract, barring any special circumstance, the obligee is not allowed to seek the performance, and the performance already performed may be claimed according to the legal doctrine of unjust enrichment, as the performance has no legal ground.

Here, it does not include cases where an obligee cannot expect the realization of the obligor's performance in light of the empirical rule or the concept of transaction in social life, rather than where it is absolutely and physically impossible to perform the obligation.

(2) In light of the empirical rule and the concept of transaction in social life, it is difficult for the Defendant (Counterclaim Plaintiff) to realize the duty to supply the software to the Plaintiff (Counterclaim Defendant) at the time of the instant supply contract, and thus, the lower court determined that the Defendant (Counterclaim Plaintiff) had the duty to return money received as partial payment from the Plaintiff (Counterclaim Defendant) in accordance with the legal doctrine of unjust enrichment to the Plaintiff (Counterclaim Defendant) in accordance with the purport that the instant supply contract, based on its reasoning, was concluded to the effect that: (a) the instant maintenance project aimed at providing maintenance and repair software; (b) the instant maintenance project was completed upon the supply of the software to D institutions, etc. by other companies; and (c) the instant project was completed upon the supply of the software for maintenance and repair prior to the conclusion of the instant supply contract; and (d) it was impossible to expect the Defendant (Counterclaim Plaintiff) to realize the duty to supply the software to the Plaintiff at the time of the instant supply contract.

Examining the relevant legal principles and records, the lower court erred by misapprehending the legal doctrine regarding the interpretation of a disposal document and the original impossibility, or failed to exhaust all necessary deliberations, contrary to what is alleged in the grounds of appeal.