Roman origins of the synallagmatic contract
Today we take it for granted that parties to an agreement can freely enter into a bargain that creates mutual and multiple obligations. This was not so in Roman law at first. For reasons not known to us now, the logical Roman mind at first recognised only a bargain in which one party incurred his own single obligation. Put differently, I can enter into an agreement only in respect of my obligation. You, on the other hand, can similarly only enter into an agreement in respect of your obligation. Roman law then, initially only recognised a unilateral undertaking to perform an obligation, entirely free of any condition or reciprocity.
Aulus would for example undertake to give Balbus 100 aureii. Balbus in turn would undertake to give Aulus his chariot. All this was done by way of simple question and answer. Balbus would ask Aulus “spondesne 100 aureii mihi dare?” (= do you solemnly promise to give me 100 aureii?), to which Aulus would answer “spondeo” (= I solemnly promise). Aulus would ask Balbus in turn “spondesne bigae tua mihi dare” (= do you solemnly promise to give me your chariot?) to which Balbus would make the required response.
Accordingly, two separate and independent obligations arose. Should it now materialise that Balbus turns out to be the typical second hand chariot salesman that does not honour his promises, it does not absolve Aulus of the obligation to hand over the 100 aureii. He has solemnly promised, and as a honest and upright Roman, he must honour his obligation. This form of bargain was called a stipulatio (stipulation, in that what is to be done is promised, or stipulated).
Initially it seems, only certa pecunia (= fixed sum of money) could be stipulated. While the origin of the institution is not known, it seems likely that the promise to give money originated with procedural promises given by a litigant who, in terms of the initial laws of the Twelve Tables, would promise to give money in the event that the case went against him. It was also used in procedural matters, for example where a child was placed under tutelage, the tutor would be required to promise to perform his duties honestly and properly. This gives rise to speculation that in the case of sale (which only developed later as a distinct and synallagmatic contract), the thing would be given, and in cases where the money (or object bartered) does not change hands immediately, the promise would be made that the money will be given, obviously at a later stage.
Where the bargain was for something to be done, it was not possible initially to promise to do what was agreed. The party obligated to do the act agreed upon, would therefore promise to pay a sum of money, on the understanding that should he not do what he agreed to, he would be held to payment of the sum promised. In later times it became possible also to promise certa res (= a specified thing).
The promise was a very solemn one, more in the nature of an oath than merely a “cross my heart and hope to die” situation. Later on, the law also recognised stipulationes where other words could be used. Such as “dabisne?” (= will you give?) and “promittisne?” (= do you promise?). While early development in Roman law did not require witnesses, it became usual to make a memorandum of what was promised, called a cautio, making proof of what was undertaken so much easier. The most valuable things in the Roman set-up, Italian land, slaves, servitudes, beasts of burden and some agricultural implements, could only be transferred by way of mancipatio, a formal act requiring at least five witnesses, all citizens in good standing and above puberty, and accompanied by all sorts of sacral acts and formulae.
The development of the stipulatio was one of the ius strictum (= the strict (ancient) law) and was therefore absolute. The promise for certa pecunia aut res was exacted by condictio, a personal action with its origins (like the stipulatio) in the ius strictum, which procedurally did not require the litigant to state the basis of the liability in his pleading before the court (his entitlement was simply the undertaking given to him). With the condictio the wronged party recovered what was promised to be given. Where the claim was for an act to be performed or something incertum, the action was an actio ex stipulatu, which procedurally did require the litigant to state in his pleading what the basis of the alleged liability was. Later law allowed a condictio incerti, but it is accepted that the appellation is a post classical invention.
Only once barter became less important during the Republic, the notion of sale developed as a single contract, although the Romans still did not call it simply sale, but emptio venditio (= purchase and sale), to some extent maintaining the distinction between what one party does and what the other does.