I am a lawyer with some knowledge of Roman law, so I'll give this question a go. Mods, be kind.
The answer to your question depends on some extent to which period of the Roman empire you're asking about. Early Roman law (753±250 BC), for example, was based on an inflexible and formal legal system called the ius civile which was only applicable to Roman citizens. The preclassical period (250±27 BC) necessitated a legal system that could deal with the empire's expansion. The ius gentium was created, which made provision for people who were within the boundaries of the Roman empire, but were not Roman citizens. It applied to disputes between foreigners and to disputes between Roman citizens and foreigners, and was more informal. During the classical period (27 BC±AD 284), in AD 212, Emperor Caracalla promulgated the constitutio Antoniniana, which theoretically extended citizenship to all the inhabitants of the Roman empire. This removed any need for different legal systems for citizens and non-citizens and the differences between the various systems therefore gradually disappeared. Etc.
Roman law had strict categories for res (things). A house would be categorised as follows: a thing in the estate (res in nostro patrimonio) and therefore a thing in commerce (res in commercio); a corporeal thing (res corporales); a composite and indivisible thing; and an immovable thing (res immobiles).
The most important distinction was whether something was res mancipi (agricultural in nature) or res nec mancipi (not). A house would be considered agricultural. In early Roman law, ownership of res mancipi could only be transferred through formal methods of conveyance, which methods were the mancipatio and the in iure cessio.
Mancipatio
The effect of mancipatio was that ownership of the things concerned passed if the things had been alienated by the owner.
Mancipatio was an ancient institution that had long been in existence when the law of the Twelve Tables was promulgated (450 BC ). It dates back to a period when coined money was not yet in use and payment was made by paying over a specific amount of copper or bronze. This explains the important part played in the transaction by the person holding the scale and the scale on which the metal was weighed.
The procedure was very formal. Both parties, the person holding the scale and the thing had to be present in front of five witnesses. The witnesses had to be Roman citizens over the age of puberty. The transferee (buyer) had to hold the thing and formally declare that the thing belonged to him and that he had bought it with the copper or bronze with which he touched the scales. The copper or bronze was then handed over to the transferor (seller).
Mancipatio was therefore originally a formal cash sale as well as a mode of transferring ownership (dominium). Coined money was only used from the fourth century BC and it expanded the application of mancipatio. The symbolism of the scales was retained, but it was simply touched with a coin and it was no longer necessary to weigh off and hand over the exact amount of money.
Ownership of res mancipi was transferred by the mancipatio procedure provided the transferor (owner), the transferee (new owner) and the witnesses were all Roman citizens (or had formally received the right to participate in Roman commerce). Further, only persons under paternal authority and res mancipi could be transferred by means of this legal procedure. The distinction between res mancipi and res nec mancipi applied from early on and remained important from the preclassical period until deep into the postclassical period.
Mancipatio was initially a cash sale and the transfer of ownership was effected simultaneously with the payment of the sum of money in cash. Once this symbolic sale had become the rule, it came to be used in this form for the transfer of ownership. The reason (causa) for the transfer of ownership by means of mancipatio could have taken a number of forms. Examples included a cash sale, a donation, the provision of a dowry, agreements arising from real security, the performance of obligations arising from a verbal contract, a legacy, et cetera.
Note that mancipatio may be regarded as an abstract mode of transfer of ownership. The reason for this statement is that if the underlying agreement (eg the contract of purchase and sale, the donation or the testamentary disposition) were to prove invalid, ownership would nevertheless pass to the recipient. The formal act as such brought about the legal consequences and the invalidity of the reason or causa did not affect the transfer of ownership.
The mancipatio procedure simply gave effect to the intention of the parties to create a real right. Mancipatio was therefore not a contract or agreement. The procedure usually consisted of the following two steps:
Agreement (eg a contract of purchase and sale)
Mancipatio
During the classical period (ie 27 BC±AD 284) the informal mode of transferring ownership (delivery = traditio) gradually supplanted the formal mancipatio in practice. Long before Justinian removed it from the classical sources and formally abolished it, mancipatio had fallen into disuse.
In iure cessio
In iure cessio (cession before the praetor) was already in existence at the time of the Twelve Tables. Like mancipatio, it was a formal act that formed part of the ius civile and could only be used by Roman citizens. All kinds of things, that is res mancipi and res nec mancipi, as well as incorporeal things, could be transferred in this way.
Both parties and the thing to be transferred (or a symbol thereof) had to appear before the praetor. The transferee took hold of the thing and formally declared that he was the owner. The transferor did not contest this and the praetor then awarded the thing to the transferee.
In iure cessio could, like mancipatio, have any reason or cause and was independent of the validity of such cause. Therefore it was also an abstract mode of transfer of ownership. It is important to remember that civil ownership only passed in the case of in iure cessio if the person who was transferring or delivering the thing was the owner. The nemo plus iuris rule (one cannot transfer more rights than one has) was therefore also applicable here.
You should bear in mind that the formal act (in iure cessio), like mancipatio, followed a preexisting agreement or cause. Therefore we can also speak of two steps.
Agreement (eg a contract of sale)
In iure cessio
This form of transfer of ownership fell into disuse at the end of the classical period and was formally abolished by Justinian.
Mancipatio and in iure cessio are considered to be abstract modes of transfer of ownership. This means that even if the underlying reason (causa) (eg the contract of sale) was invalid, ownership still passed.
Ask if you have any more questions.
Source: Foundations of the South African Law by Prof H van den Bergh et al.
Interesting question and I have no answer. Just questions of my own.