Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Contracts concluded between the state and a burgess, particularly the obligation given by those who became sureties for a payment to the state (-praevides-, -praedes-), were valid without further formality. On the other hand, contracts between private persons under ordinary circumstances gave no claim for legal aid on the part of the state. The only protection of the creditor was the debtor's word of honour which was held in high esteem after the wont of merchants, and possibly also, in those frequent cases where an oath had been added, the fear of the gods who avenged perjury.
The only contracts legally actionable were those of betrothal (the effect of which was that the father, in the event of his failing to give the promised bride, had to furnish satisfaction and compensation), of purchase (-mancipatio-), and of loan (-nexum-). A purchase was held to be legally concluded when the seller delivered the article purchased into the hand of the buyer (-mancipare-) and the buyer at the same time paid to the seller the stipulated price in presence of witnesses. This was done, after copper superseded sheep and cattle as the regular standard of value, by weighing out the stipulated quantity of copper in a balance adjusted by a neutral person.(4)
4. The -mancipatio- in its developed form must have been more recent than the Servian reform, as the selection of mancipable objects, which had for its aim the fixing of agricultural property, shows, and as even tradition must have assumed, for it makes Servius the inventor of the balance. But in its origin the -mancipatio- must be far more ancient; for it primarily applies only to objects which are acquired by grasping with the hand, and must therefore in its earliest form have belonged to the epoch when property consisted essentially in slaves and cattle (-familia pecuniaque-).
The enumeration of those objects which had to be acquired by -mancipatio-, falls accordingly to be ranked as a Servian innovation; the -mancipatio- itself, and consequently the use also of the balance and of copper, are older. Beyond doubt -mancipatio- was originally the universal form of purchase, and occurred in the case of all articles even after the Servian reform; it was only a misunderstanding of later ages which put upon the rule, that certain articles had to be transferred by -mancipatio-, the construction that these articles only and no others could be so transferred.
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Reference address : https://www.ellopos.net/elpenor/rome/1-11-law-justice.asp?pg=11