In order for something to be just, fair, or equitable, you have to weigh it against something else.
It's just to offer a fair wage because you're weighing the wage against the work performed.
It seems like you're conflating justice with recompense. Compensation is fair, but fairness isn't compensation. Compensation is just, but justice isn't compensation.
An old definition of justice is "Rendering to each one his due," or "Rendering to each one his right." If you injure or employ, then it is just to repay or pay (respectively), but justice isn't merely transactional. For example, the just man will defend himself, his family, and his country against harm. In this case justice is a matter of duty, not recompense.
(Note, though, that promise-keeping is a form of justice-as-recompense. The fulfillment of a promise is weighed against the promise itself (or the "man's word"). A promise is the creation of an obligation which must in turn be fulfilled.)
It's just to fulfill one's contractual obligations after the other party has,
According to such a notion no contractual obligation would ever be fulfilled.
...but you can't conflate that with keeping a promise. Fulfilling your end of a contract is fulfilling a promise, but fulfilling a promise isn't necessarily fulfilling your end of a contract.
A promise is a one-way obligation; a contract is a two-way obligation (or three-way, or four-way, etc.). You might define a contract as a legally enforceable set of agreed-upon promises. Either way, fulfilling a promise is just, and fulfilling a contract is just, and for much the same reason.
If I make an unsolicited promise to you that I'm going to wash your car, it isn't "just" to keep that promise because it isn't compared with anything.
Of course it's just. I addressed your compensatory notion above.
It feels like you're trying to conflate "just" with anything "good". But lots of things are good that are unjust. Being generous, for one.
All that is just is good but not all that is good is just.
It's good to give people more than you owe them, like tipping well, right? But that isn't just. How can being just be intrinsically good when it's good to be unjust? If I offer you more than a fair wage, am I being bad because I am acting unjustly?
Supererogation is certainly not unjust.
Now like you said, we're talking about retributive justice, so why is your example the opposite of that?
The reason we moved to the topic of justice is because you asserted that justice itself has no intrinsic value. I gave an example of non-retributive justice in order to establish a non-controversial ground/genus by which we could then determine whether retributive justice is, in fact, just. (I said this in different words
a long time ago.)
I believe
Tinker's response was much better. Rather than questioning justice itself, he questioned the justice of an act. His standing argument is that the act of eternal punishment is unjust because it is non-rehabilitative.