James, I think we're almost on the same page here. It's not that labor produces value in excess of the value of wage that makes capitalism unjust as the worker is indeed paid the value of her labor power. the wage does reflect the value of labor power. Where's the injustice?
But the value with which workers as a whole are paid is value that has already been appropriated not from this worker per se but other workers, the working class in general. Yet what matters sadly is the justice of the relationship between the individual capitalist and worker. So where's the injustice?
Capitalists pay with appropriated value from the working class as a whole for labor power from which they appropriate value yet again. That is, from the perspective of dynamic macroeconomics and the class relationship, there is no real exchange at all but appropriation plain and simple.
From the dynamic and totalistic macroeconomic or "dialectical" perspective the exchange relation is revealed as an unjust one of appropriation.
It's because we can't take that perspective on the apparent exchange relationship that capitalism cannot be critiqued from within as unjust.
Hope this makes sense, and I am trying to make sense of it.
Marx's argument in chapter 24 is quite complicated, and seems to me not to have been understood.
Perhaps though I am closer to Allen Wood (who has been saying pretty radical things as a tenured philosophy prof at Stanford) than you are on this question, as I don't think Marx thinks we can get very far with a critique of capitalism in terms of a theory of justice. Which is not to say that he accepts that it is just, but that a negative judgement depends on standards foreign to capitalism itself and thus cannot be presented as an immanent critique.
Rakesh