雙務契約給付返還不當得利法之適用

給付不當得利預設單方給付之返還，與契約解除法律效果之規定以雙務契約為設想對象，大相逕庭。契約無效之返還與契約解除後之法律效果，均為回復原狀，請求的目的在返還給付，而非除去受領人所現存之利益，目標一致。給付不當得利請求權排除、所受利益不存在等規定無差別適用於無效雙務契約之返還，牴觸契約回復原狀之本旨，有待商榷。最高法院在不僅在契約之解除，認為當事人亦得依不當得利請求返還，在合意解除契約、解除條件成就發生給付返還關係之情形，偏好不當得利之適用，不符合當事人意思自主及雙務契約返還之特殊性。

關鍵字

無效雙務契約；不當得利；契約之解除；危險負擔；合意解消；解除條件

並列摘要

The rules of performance-based restitution are made for the restoration of the one-sided enrichment. On the contrary, the provisions on the legal effects of termination of contract are designed to recover the performance of bilateral contracts. Restitution of the invalid contract and the legal effect of the termination of contract are both to restore the status quo. Both claims share the same goal. The subject matter of both claims is giving back what recipient received from the performer, not taking away the benefits left for the recipient. It will violate the essence of restoration of bilateral contracts, if the rules on the preclusion of the claims of unjust enrichment and the rules for the defense of change of position apply indifferently. Some cases of our supreme court hold that, the terminating party is also entitled to claim back what was performed under contract based on the law of unjust enrichment. Besides that, in cases of the agreement to terminate contractual relation and the satisfaction of the condition subsequent, it prefers the application of unjust enrichment, which is inconsistent with the parties' autonomy and the essence of bilateral contract.