There are certainly many sorts of contracts. Contracts in general grant moral powers to the parties to imposes obligations upon each other, this at least shall be an initial position.
The wage contract seems to be essentially different from other contracts of exchange, such as sale and rent. A contract of sale exchanges ownership of property in return for something else. A contract of rent exchanges the temporary possession and use of something while maintaining the ownership of it.
A wage contract however is essentially different from these sorts of contracts. A person is not he sort of thing that can be sold nor can a person even possibly hand over possession of himself for use. However, in a certain way a wage contract is analogous to a contract for rent, but they are still essentially different.
In the wage contract, a laborer offers his powers to actualize some ends to the other party. He does not offer these as something the other party can take possession of. Rather the laborer as one capable of directing his will and powers to certain ends proposes to will what the other party will, that is the ends which the other party desires. Whereas in a contract of rent, the part takes hold of the thing and orders it himself.
The wage contract then represents a certain coordination of wills. As I have spoken before, a thing is valuable with relation to usability, but there is a certain qualification with respect to the value of labor. A laborer cannot be used in the same way as an inanimate thing. However, we can see here that usability under the aspect of an ability to coordinate will is here.
A person who has a great ability to do all sorts of things may have a great intrinsic value in virtue of those powers. However, if he is obstinate and unwilling to work with or for others, his labor is worthless in a way that uranium is worthless to those unable to use it. Since, he refuses to coordinate his will with others, his powers though great are worthless.
In any case, the wage contract is essentially different from the other contracts of sale or rent. The conditions for justice are determined by the nature of contract. Justice under a wage contract is different than that under sale or rent.