1. Solutio per aes et libram and acceptilatio as actus contrarii

The Latin word for performance/fulfilment of an obligation was "solutio", acceptance in lieu of fulfilment (or substituted performance) was referred to as datio in solutum.

This may appear to be a strange terminology, because literally speaking, the verb "solvere" means to unbind, to untie (somebody). This word and all its derivations possess a distinctly archaic flavour. They take us back to the days when an obligatio was not merely a vinculum iuris but a physical bond, through which a pledge-like power of seizure was established over the body of the person Hable for a wrong.^[3866] In the case of what came to be called "contract", one person subjected himself to this power of seizure by means of a formal transaction. Naturally, however, there had to be some way of bringing to an end this uncomfortable sojourn in the "creditor's" dungeons. Neither of the two parties concerned normally had any interest in bringing matters to a head: to a sale of the "debtor" trans Tiberim or perhaps even to his being cut into pieces.^[3867] If we look, for instance, at nexum as one of the oldest liability transactions,^[3868] we see that the "debtor" was supposed to redeem himself by repaying, within a certain time, a specific sum that had been lent to him. It was only where he failed to do so that the question of his liability—that is, of the creditor's power of seizure—became relevant. But (informal) payment as such, although in a substantial sense it satisfied the "creditor", did not release the person liable from his bondage. The obligation had been created by a strictly formal act, and thus it could be "solved" only by another formal act, which, as a rule, had to correspond to the former. Thus, where the person liable had assumed his obligation by way of nexum (a solemn act per aes et libram), he could disentangle himself only by performing another act involving copper, scales and witnesses: the solutio per aes et libram (or nexi liberatio).^[3869] With the rise of the contract verbis (stipulation in particular), acceptilatio was developed as an oral form of dissolving oral obligations. Solutio per aes et libram and acceptilatio were thus devised as symmetrical actus contrarii to the transactions they were intended to discharge; they provide a good illustration of the ancient idea (featuring particularly prominently in the religious and magical spheres)^[3870] that what has once been done cannot be undone but by a corresponding act^[3871]—and since all (legally) relevant acts in ancient times were formal, the actus contrarius necessarily had to be formal too.

More on the topic 1. Solutio per aes et libram and acceptilatio as actus contrarii:

1. Formal release by way of solutio per aes et libram and acceptilatio
2. 2. The rise of informal solutio
3. Solutio Indebiti
4. Undue payment (solutio indebiti)
5. I. SOLUTIO
6. "Solutio propria", "in praecisa forma et specie obligationis"[3885] (to use the terminology of the European ius commune) has always been, and still is, the most important way of terminating obligations.
7. OTHER FORMS OF "SOLUTIO IMPROPRIA"
8. Forms of testaments
9. The limitations of sponsio
10. INDEX
11. Dies a quo
12. Performance
13. Correality and solidarity
14. 4. DISCHARGE OF OBLIGATIONS
15. Index
16. The nature of mutuum
17. Release
18. Datio in solutum
19. ‘Quasi-contract’ is an unsatisfactory term applied to certain specific obligations which did not arise from contract or delict but were legally enforceable.
20. Informal release