In an exchange, I get something other than what I give. A bike for a guitar. The things change owners. But something stays with me: the value. Before, I had a bike worth 200 euros. Now I have a guitar of the same value. In value, the things are equal, however different they look.

What if the exchange is very uneven?

Roman law had a rule for this. If someone got less than half the value, he could cancel the contract. This rule follows from the nature of contract. For in a contract, I should remain the same owner in terms of value.

Much worse still is a contract about something one may not give away at all. For example, about one’s own freedom. This harm is not just great. It is beyond all measure.

Single pledges:

A single pledge is not yet the whole contract. One person pledges to do something. The other pledges to accept it. That is why people have called them one-sided contracts.

The Romans sorted contracts in many ways. Often by outward features, such as their form. In doing so, they mix different things. What belongs to the nature of contract stands next to rules for the court. This is shallow and clouds the concept of right.

Summary: In an exchange, each remains the owner of the same value. So an exchange must not be grossly uneven. A contract about something you may not give away has no force at all.

§ 494. Just as in stipulation the substantial element of the contract is distinguished from the performance as the real expression, which is reduced to a mere consequence, so too, with respect to the thing or the performance, the distinction is posited between its immediate specific character and its substantial element, value, in which that qualitative determinacy is transformed into quantitative determinacy; a piece of property is thereby made comparable with another and can be equated with something qualitatively wholly heterogeneous. Thus it is posited in general as an abstract, universal thing.

§ 494. Just as in stipulation the substantial element of the contract is distinguished from the performance as the external element, which is reduced to a mere consequence, so too, with respect to the thing or the performance, the distinction is posited between its immediate specific character and its substantial element, the value, in which that qualitative determinacy is transformed into quantitative determinacy; a piece of property thereby becomes comparable with another and can be equated with something qualitatively wholly heterogeneous. Thus it is posited in general as an abstract, universal thing.