論減損義務對實際履行請求權的限制

Fetter on Specific Relief by Duty of Mitigation

大陸和臺灣的履行障礙法，均以實際履行作為優先的救濟手段，原則上當事人得為自己的利益計算而選擇該等救濟方式。然而在某些極端案型中，當事人不合理地堅持實際履行請求權可能引發投機、嚴重的資源浪費等問題，此時否定實際履行請求權的其他規則，如履行不能、情事變更常常無法解決之。譬如金錢債務原則上不生履行不能問題，若金錢之債的債權人（如出租人）堅持實際履行，那麼給付不能規則便束手無策；當事人終止合同的需求又常因一般商業風險而起，此際情事變更亦不敷用。故只能以減損規則限制履行請求權。
形式上，大陸的實定法之下，減損義務並非隨同明確的毀約表示立即發生，而僅在例外情形下產生限制功能；同時，並無必要為了避免非違約方遭受同時受領新舊 2 份合同之給付或者自陷於違約的危險，而構造出一種適時解除義務。判斷何時應當觸發減損義務的限制功能，應當以堅持實際履行是否造成嚴重的浪費為判斷標準。臺灣民法並無一般的減損義務條款，但並不意味著不會產生類似問題，此時經由民法第 217 條的與有過失條款可實現相同效果。在這個意義上，關於限制實際履行請求權之實質標準的討論，於臺灣法而言亦應有一定價值。

Specific performance is the core of the system of non-performance law both in Mainland China and Taiwan, and parties are entitled to choose this remedy on account of their own benefits and interests. But under some extreme cases, that parties insist on specific relief unreasonably may lead to speculation by non-defaulting party or enormous waste of resource, so we have to find some norms to limit the claim. Some other tools, however, such as impracticability, rebus sic stantibus cannot achieve this goal. For example, monetary obligation will not extinguish out of impracticability, so when the obligee (such as a lessor) of that obligation insist on enforcement of the debt, the other party cannot claim impracticability in order to get rid of it. General commercial risks which induce parties to end their contract will also exclude rebus sic stantibus. As a result, rule of mitigation has to play a role.
In terms of Mainland China’s positive law, technically, duty of mitigation does not arise the moment the defaulting party sends a notice of repudiation or breach, but fetters the claim of specific performance only under extreme circumstances. To avoid the risk of accepting double deliveries from both the original and the substitute contract, there is no need to construct a duty to terminate the contract in time. Substantively, the test on deciding when a claim of specific relief should be turned down is whether the insistence would cause unacceptable wastefulness. There exists no general clause of mitigation in civil code of Taiwan which, however, does not mean similar problem does not exist neither. Article 217 (comparative negligence) of civil code Taiwan can realize the same effect of　limitation. No matter how different the legal systems are, people in diverse communities are confronted with similar problems. Fetter on specific performance by duty of mitigation may echo with the need of a highly commercialized world.