本文对不当得利返还，即不当得利法律效果相关问题进行研究。本文不采取非统一说对给付型、权益侵害型、费用支出型和求偿型不当得利的分类分别探讨不当得利的法律效果。全文按照三条线索展开：第一条线索是按照返还客体-返还范围-返还地位来建构整体框架。第二条线索是区分一般不当得利返还与双务合同失败后的返还。第三条线索是在双务合同失败返还内部对双务合同无效、被撤销与解除后返还的具体问题的讨论，试图建立统一的双务合同失败返还规则。就返还客体而言，《民法典》第985条规定的不当得利返还请求权的客体——“取得的利益”，不仅指“得利”，还包含“本于得利而更有所取得的利益”。“本于得利而更有所取得的利益”包含三类：孳息及使用利益、基于权利的所得、原物的代偿。在返还方式上，当得利可移转时，需要原状返还；当得利不可移转时，或原初得利虽可移转、其后变得不可移转时，则需要进行价值返还。价值返还的计算采取客观说，强迫得利问题不需要通过价值计算的主观说来解决。相较于一般不当得利返还客体，双务合同失败返还客体呈现出特殊性：孳息与使用利益的返还采取相互抵销模式；双务合同失败后，需要处理返还标的物增值或贬值问题；双务合同失败后的价值返还通常以合同价格为准计算。就返还范围而言，核心问题是《民法典》第986条的得利丧失抗辩规则。除了得利丧失与善意要件之外，解释上应当加上因果关系要件。得利人证明得利丧失抗辩后，失利人可以提出得利人对于不当得利的发生具有过错的再抗辩；得利人可以进一步提出失利人对于不当得利发生具有更大过错的再再抗辩。双务合同失败后返还关系中的得利丧失抗辩问题有两种处理路径：适用或类推适用《民法典》第986条；或者发展双务合同失败返还中的得利丧失抗辩规则。就返还地位而言，不当得利返还请求权属于债权性质，通常只能与其他债权平等受偿，不具有优先地位。但是，某些情形下的不当得利返还请求权在破产与强制执行程序中具有优先地位，典型表现是错误汇款返还请求权。双务合同失败后的返还与一般不当得利返还存在共通之处，也存在区别。宜将《民法典》第157条解释为双务合同无效、被撤销后相对于一般不当得利返还的特别规定，构成独立的请求权基础，但对于不能从该条解释出且与一般不当得利返还相同的规则，则可以适用《民法典》不当得利一章的规定。合同解除后的返还适用《民法典》第566条，没有规定时类推适用不当得利一章的规定。
The topic of this paper is restitution on unjust enrichment. Unjust enrichment is classified as four types: the claimant renders a performance to the defendant (Leistung); the defendant encroaches on the claimant’s property (Eingriff); the claimant incurs expense in improving the defendant’s property (Verwendungen), and the claimant pays the defendant’s debt (Rückgriff). This paper does not adopt this classification in the discussion of the effects of unjust enrichment. This paper follows three clues. The first clue is in accordance with the object – scope – status. The second clue distinct restitution on failed bilateral contracts from restitution on general unjust enrichment. The third clue is that restitution on avoidance and termination should be uniform.The object of restitution on unjust enrichment set out in §985 is enrichment, which contains the immediate enrichment and subsequent enrichment. The subsequent enrichment includes fruits, use, benefits from realization of right, and substitute. A transfer in specie is the primary measure of restitution if the enrichment takes the form of a transferable asset. Where the enrichment does not take the form of an asset that is transferable, or the asset can no longer be transferred, the measure of restitution is by paying the monetary value of the enrichment. The monetary value of the enrichment should be determined by the objective value. Imposed benefit is not an issue in the second stage of whether restitution should be qualified by subjective devaluation.Compared with restitution on general unjust enrichment, objects of restitution based on failed bilateral contracts have some features. Fruits and use will be offset and no longer need to make restitution; the appreciation and devaluation of the transferrable asset should be considered; generally, the monetary value should be measured according to the contract price.The core issue of the range of restitution is the change of position defence regulated by §986 of the Chinese Civil Code. Besides the elements of disenrichment and good faith, causation should be required. The plaintiff can disqualify the defence by proving the defendant’s fault in causing unjust enrichment. Conversely, the defendant can break off the disqualification by showing that the plaintiff is more responsible than him for the circumstances giving rise to unjust enrichment. There are two ways to deal with the issue of change of position in the restitution of failed bilateral contracts. The first way is to apply or analogically apply§986 of the Civil Code; the second way is to develop an independent change of position defence rule in the restitution based on failed bilateral contracts.Restitution claim based on unjust enrichment is personal and should follow the pari passu principle. However, sometimes restitution on unjust enrichment in cases of mistaken payments can be given priority. Personal restitution on failed bilateral contracts cannot be given priority.There are connections and distinctions between restitution on general unjust enrichment and restitution on failed bilateral contracts. §157 can be explained as specific rule when bilateral contracts are void or rescinded compared with restitution on general unjust enrichment and thus is an independent basis of claim. However, for the rules that cannot be interpreted from this article, Chapter 29 of the Chinese Civil Code can be applied. Restitution after termination of contracts is subject to §566 of the Civil Code. If there is no provision, Chapter 29 shall be applied by analogy.
