1. Indebitum solutum

A sells his horse to B. Some weeks later, he duly delivers the animal to the purchaser. When he sues him for the purchase price, the contract of sale turns out to have been invalid.

Obviously, under these circumstances, A must be able to recover the horse from B: for it would be blatantly unfair if B were allowed to keep what he has acquired without having to render counterperformance. A's claim can, however, be based neither on delict nor on contract; B, by accepting delivery of the animal, did not act unlawfully, and A, by making delivery, intended to discharge his own contractual obligation, not to create one on the part of B (to return the horse).^[4282] Occasionally, A will still be able to institute the rei vindicatio. In modern French law, for example, it is the contract of sale that transfers ownership;^[4283] if the sale is invalid, ownership remains with the vendor, and B merely acquires possession. Very often, however, the vendor can no longer avail himself of an actio in rem. In modern German law, the transfer of ownership constitutes a separate transaction (separate, that is, from the underlying contract of sale),^[4284] which, moreover, has to be evaluated "abstractly": invalidity of the contract of sale does not, as a rule, affect the transfer of ownership.^{[4285] [4286] [4287] In Roman law, too, A would have lost his real right: horses were res mancipi, and transfer of (quintary) ownership therefore required either mancipatio or in iurc cessio. Both acts were abstract and remained valid irrespective of the fate of the obligatory transaction they were supposed to discharge.8}

A special remedy is thus required, a remedy in personam and based on the fact that the purchaser has received what subsequently turned out to be "indebitum": a performance that was never owed to him and that he therefore has no right to keep. This remedy was instituted by the Roman lawyers and it is usually referred to as condictio indebiti. Over the centuries, it has become one of the cornerstones of our modern law of unjustified enrichment. Whence the term, "condictio", to designate the claim?

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