潛伏於雙務合同中的等價性

雙務合同涉及給付的牽連關係，但理論上始終存在一種有待明確的經濟意義上的等價性訴求。這一訴求事實上隱含著制度史與比較法知識背景下的多重指向。在雙務合同的概念及其相關牽連性制度中，基於歷史的演進，作為要素的等價性發生了脫落。但在附負擔贈與和締約嚴肅性的判斷中，等價性仍具有識別與定性的線索性要件意義。當代的等價性主張集中在顯失公平與情事變更制度之中，但這一等價性主張不觸及雙務合同的概念，而是表現為合同中的常素，並體現出給付關係外部調整的現代性與謹慎性。據此，顯失公平判斷的動態體系化主張仍有商榷餘地；情事變更制度下，共同認識錯誤問題應另作安排，重新協商的內容及其違反後的處理可以作更精細的設置，而合同因情事變更解除之後應考慮履行利益損害賠償的可能性。

The bilateral contract involves the synallagmatic relationship of payments, but in theory there are always propositions referring to economic equivalent to be clarified. These propositions imply actually multiple orientations in the context of institutional history and comparative law. In the concept of bilateral contract and its related synallagmatic system, based on the evolution of institution, the equivalence as an essentialia negotii has fallen off. However, in the identification of the favour contract attached burden and the seriousness of contracting, the equivalence can still provide clues for judgement. The contemporary propositions about equivalence are concentrated in the institutions of explicit unfairness and change of circumstances. Such claims do not touch on the concept of the bilateral contr act. They are however expressed as naturalia negotii in the bilateral contract, and reflect the modern and prudent characters in the external adjustment of the payment relationship. According to this, there is still necessity for reflection on the dynamic systematized judgement of unfairness; under the institution of change of circumstances, the problem of common mistake should be arranged separately, the content of renegotiation and its enforcement can be set more specifically, and the possibility of the damages in lieu of performance should be considered, after the contact has been terminated.