UE EQ1: What is the relationship between contract law and the law of unjust enrichment?
UE EQ1：合同法与不当得利法之间的关系是什么？
思考：
The relationship between contract law and the law of unjust enrichment is one of the most fundamental and dynamic interactions within the law of obligations. While they are distinct legal disciplines with different objectives, they operate in a complementary rather than conflicting manner. Contract law’s primary function is to enforce consensual agreements and protect future expectations, whereas the law of unjust enrichment is concerned with reversing non-consensual or defective transfers of value from the past. Their relationship is best understood by examining the general principle of contractual primacy and the various "interfaces" where the law of unjust enrichment steps in when the contractual regime fails or is absent.
合同法与不当得利法之间的关系是债权法中最基本且最具活力的互动之一。虽然它们是目标不同的独立法律学科，但它们是互补而非冲突的。合同法的主要功能是执行双方自愿达成的协议并保护未来的预期利益，而不当得利法则关注于纠正过去非自愿或有缺陷的价值转移。理解它们的关系，最好从合同优先原则以及不当得利法在合同制度失败或缺失时介入的各种“接口”入手。
The foundational principle governing their relationship is that a valid and subsisting contract bars a claim in unjust enrichment. This principle, often referred to as subsidiarity, dictates that where parties have a valid agreement that governs their rights and obligations concerning a particular benefit, the law of contract is the exclusive regime for resolving disputes. The contract itself provides the justa causa, or legal basis, for the defendant's enrichment, thereby negating the third essential element of an unjust enrichment claim. A party cannot simply bypass an inconvenient contractual term, such as an unprofitable price, by pleading an unjust enrichment claim. This ensures that the autonomy of the contracting parties and the certainty of their commercial arrangements are respected.
支配两者关系的基本原则是，有效且持续存在的合同排除不当得利的请求。该原则通常被称为补充性原则，规定当双方有一份有效协议来管理其关于特定利益的权利和义务时，合同法是解决争议的唯一法律体系。合同本身提供了被告得利的合法依据（justa causa），从而否定了不当得利请求的第三个必要要素。任何一方不能仅仅通过提出不当得利请求来规避合同中不利的条款，例如不盈利的价格。这确保了合同双方的自主权和其商业安排的确定性得到尊重。
The true interaction between the two areas of law occurs at their interface, primarily in situations where the contractual framework is either defective from the outset or has ceased to operate. It is in these "gaps" that the law of unjust enrichment performs its crucial role.
两者法律领域的真正交互发生在其交界处，主要是在合同框架从一开始就存在缺陷或已停止运作的情况下。正是在这些“空白”处，不当得利法发挥其关键作用。
First, where a purported contract is void ab initio, for reasons such as fundamental mistake, incapacity, or illegality, there is no legal basis to justify any transfer of value made pursuant to it. In such cases, the law of unjust enrichment intervenes to reverse the transfer. For instance, in cases concerning interest rate swaps entered into by local authorities, which were held to be ultra vires and thus void, the courts allowed the banks to recover payments made under the purported contracts. The unjust factor here was a total failure of consideration, as the "consideration"—the reciprocal legal obligations—never existed (Guinness Mahon & Co v Kensington and Chelsea Royal LBC). Similarly, where work is done in anticipation of a contract that never materialises due to uncertainty, restitutionary relief in the form of quantum meruit is available, not on the basis of a fictional contract, but on the ground of unjust enrichment (British Steel v Cleveland Bridge).
首先，当所谓的合同自始无效，原因如根本性错误、无行为能力或违法时，依据该合同进行的任何价值转移均无法律依据。在此类情况下，不当得利法介入以撤销该转移。例如，在涉及地方当局签订的利率互换合同案件中，该合同被认定为超越权限因此无效，法院允许银行追回根据该所谓合同支付的款项。不当因素在于对价完全失败，因为“对价”——相互的法律义务——从未存在（Guinness Mahon & Co 诉 Kensington and Chelsea Royal LBC 案）。同样地，当因不确定性导致合同未能实现而提前完成工作时，可通过量力而为的返还救济获得补偿，这不是基于虚构合同，而是基于不当得利（British Steel 诉 Cleveland Bridge 案）。
Second, where a contract is voidable due to a vitiating factor like duress, undue influence, or misrepresentation, the innocent party has the right to elect to rescind the contract. Once rescission occurs, the contract is treated as if it never existed, again removing the legal basis for any enrichment conferred. The restitutionary claim then operates to restore the parties to their pre-contractual positions.
其次，当合同因胁迫、不当影响或虚假陈述等瑕疵因素而可撤销时，无辜方有权选择撤销合同。一旦撤销发生，合同即被视为从未存在过，同样消除了任何利益转移的法律基础。此时，返还请求权起作用，将双方恢复到合同前的状态。
Third, the relationship is vividly illustrated in the context of terminated contracts. Historically, the common law's rigid "loss lies where it falls" rule for frustrated contracts produced significant injustice. The landmark decision in Fibrosa v Fairbairn was a pivotal moment, where the law of unjust enrichment, through the unjust factor of total failure of consideration, intervened to allow the recovery of a pre-payment where no part of the bargained-for performance was received. This common law development was later codified and rationalised by statute (in Hong Kong, the Law Amendment and Reform (Consolidation) Ordinance), which provides a more sophisticated regime for restitution and loss apportionment, but one that is still fundamentally restitutionary in spirit.
第三，这种关系在终止合同的情境中表现得尤为鲜明。历史上，普通法中对受挫合同适用的僵硬“损失自负”规则导致了重大不公。Fibrosa 诉 Fairbairn 案的里程碑式判决成为关键时刻，不当得利法通过“对价完全失败”的不当因素介入，允许在未收到任何约定履行部分的情况下追回预付款。这一普通法的发展后来被法规（在香港为《法律修订及改革（汇编）条例》）予以成文和合理化，提供了更为复杂的返还和损失分摊制度，但其本质仍然是返还性质的。
The most contentious and evolving interface lies in contracts terminated for breach. While the innocent party has an undoubted right to claim damages in contract law, the question arises whether they can alternatively elect to bring a claim in unjust enrichment for restitution. This choice becomes particularly attractive when the innocent party is in a "losing bargain", where contractual damages would yield little or nothing. This scenario pits two fundamental legal policies against each other: the policy of reversing an unjust gain by the contract-breaker versus the policy of upholding the contractual allocation of risk agreed by the parties.
最具争议且不断发展的交汇点在于因违约而终止的合同。虽然无过错方在合同法中无疑有权要求赔偿损失，但问题在于他们是否可以选择以不当得利提出返还请求。当无过错方处于“亏本交易”时，这种选择尤为具有吸引力，因为合同赔偿可能几乎无效或毫无收益。这种情形使两项基本法律政策相互对立：一方面是撤销违约方不当得利的政策，另一方面是维护双方约定的合同风险分配的政策。
The Australian High Court's decision in Mann v Paterson provides the most thorough modern analysis of this conflict. The majority sought a compromise: for completed stages of a divisible contract, the innocent party is confined to a claim in contract for the accrued debt. However, for uncompleted stages, the party may elect to claim restitution. Crucially, to prevent the subversion of the contractual risk allocation, this restitutionary claim is, as a general rule, capped by the contract price. The minority, in contrast, took a stricter view, arguing that the existence of an enforceable contract should entirely preclude any recourse to unjust enrichment for work performed, limiting the plaintiff solely to a claim for contractual damages. While Mann v Paterson is not binding in Hong Kong, its reasoning highlights the deep theoretical tensions and the ongoing judicial effort to define the precise boundary between the two legal domains.
澳大利亚高等法院在 Mann 诉 Paterson 案中的判决提供了对这一冲突最为详尽的现代分析。多数意见寻求一种折衷方案：对于可分割合同中已完成的阶段，无辜方仅限于基于合同的已产生债务请求。然而，对于未完成的阶段，当事方可以选择请求返还不当得利。关键在于，为防止合同风险分配被破坏，这种返还请求通常以合同价格为上限。相比之下，少数意见持更严格的观点，认为存在可执行合同应完全排除因已完成工作而诉诸不当得利的可能，原告仅限于请求合同损害赔偿。尽管 Mann 诉 Paterson 案在香港不具约束力，但其推理凸显了两种法律领域之间深刻的理论张力以及司法界持续努力界定二者精确界限的过程。
In conclusion, the relationship between contract law and the law of unjust enrichment is symbiotic. They are not competing but complementary regimes. Contract law is the primary, governing framework for consensual transactions. Unjust enrichment operates as an essential, secondary body of law, stepping in to provide restitutionary justice where the contractual framework is vitiated, absent, or has been legitimately terminated. While the boundary between them is mostly clear, the interaction in post-breach scenarios continues to be a fertile ground for legal development, as courts strive to reconcile the foundational principles of both disciplines to achieve a just and coherent outcome.
总之，合同法与不当得利法之间的关系是共生的。它们不是竞争关系，而是互补的法律体系。合同法是自愿交易的主要治理框架。不当得利法作为一个重要的辅助法律体系，在合同框架被破坏、缺失或合法终止时介入，提供返还性救济。虽然两者的界限大多清晰，但在违约后的情形中，两者的互动仍是法律发展的沃土，法院努力调和两者的基本原则，以实现公正且连贯的结果。