부당이득과 사용이익 - 피셔와 카메레르의 견해를 중심으로 -

Assuming that the buyer or borrower acquires and starts occupying the object under a sales contract or lease contract, but later becomes invalid, the unjust enrichment debtor who occupied the object may be considered to have been acquiring the profit from the use of the object without legal cause during the period of possession. It may also be said that the gains from the use of the land were obtained even if another person's land was mistaken for his own land and used it as a parking lot. In this case, according to the unjust enrichment law (Article 741 of the Civil Act), the profits obtained by the unjust enrichment debtor shall be returned. Based on the above, Fischer's argument can be summarized as follows. Article 812 of the BGB is amended in accordance with Article 818, Paragraph 3. As a result, the subject of the obligation to return is not regarded as the 'benefit' of Article 812, which is interpreted as the 'property difference = existing profit' of the unfair gain debtor. The reason for this interpretation is that the unfair gain debtor should not reduce his property by fulfilling the obligation to return it than in the case of no legal cause. Fischer's theory of difference can be said to be a view based on Roman law and deliberation in the BGB compilation process, which limits the obligation to return to existing profits. Fischer, on the one hand, calls the right to claim the return of unfair gains a claim for the value of the property, that is, the subject of the obligation to return the property the 'property difference', but on the other hand, argues that in many cases the calculation of the difference is unnecessary if the return of a specific object is a problem. This contradiction can be seen as an important problem with Fischer's theory of difference. On this point, Fisher himself pointed out the problem, "Can we exercise the right to claim property value for the return of specific objects?" and then virtually denied it, calling it a future review task. In any case, Ficsher's theory of difference can be considered to have influenced German precedents since it was published in 1913. On the other hand, on the premise of criticism of the difference theory and precedent, Caemmerer considers the subject of the obligation to return in two stages as follows. First, the object of the obligation to return is 'acquired', and then, if the 'acquired' by the unfair gain debtor is extinguished in good faith, the object of the obligation to return is exceptionally reduced to 'benefit = property difference = existing profit'. Therefore, Caemmerer's typology ultimately has no difference from the difference theory, and like the difference theory, it can be said that it is a view based on the deliberations in Roman law and the BGB compilation process. The two-stage structure taken by Caemmerer's typology can be evaluated as consistent with the view of the BGB compilation process that the unfair gain debtor is responsible for proving the extinction of the gain. For this reason, it can be evaluated that Caemmerer's typology is closer to the view of the BGB compilation process than Fischer's theory of difference.