This is particularly important in terms of the lack of overlap between the condictio indebiti and condictio causa data causa non secuta.
For example, the condictio indebiti lies where the inferred purpose is to discharge a debt that does not exist.
25) It also explains why it is not wholly irrational for Scots (26) and South African law (27) to have a requirement the claimant show a mistake in their condictio indebiti actions; the mistake requirements are not bizarre imports from English law, but have a respectable Roman vintage, (28) and the error requirement, which must be a mistake as to liability, is tied into the requirement that no debt or other valid obligation to pay exists.
The condictio causa data causa non secuta (sometimes shortened to condictio causa data, or condictio cd) is not therefore an equivalent to the failure of consideration unjust factor, despite the similarities, and the condictio indebiti is not--despite the similarities--a mistake action.
Properly understood, where a party believes there to be a contract, he performs to discharge the obligation--not to receive counter-performance--and so the condictio indebiti exists if it is a void obligation, and only the condictio indebiti.
63) She reanalyzes the condictio indebiti in terms of different unjust factors, including failure of consideration.
He argued that in South Africa under the condictio indebiti, the mistake must be a mistake as to liability; in English law it can be a mistake as to pretty much anything at all, so long as it causes the payment.
In one sense of the word "reason" this is right, but what du Plessis is expressing is that the essence of the cause of action in the condictio indebiti is not the mistake or the lack of freely chosen action.
Part II cleared the ground, establishing that in contrast to the common law's unjust factors, the civilian condictio claims are mutually exclusive and never lie concurrently because the condictio indebiti and the condictio causa data causa non secuta, for example, deal with the consequences of the failure of different legal bases or underpinning purposes of the transfer--and there is (or should be) only ever one legally effective purpose.
On the assumption Garland civilianizes the law, therefore, the first limb should be in effect modelled as a condictio indebiti (although neither McInnes nor the case law use this language), dependent on the failure of a purpose to discharge an obligation or make a gift.