It simply makes no sense in the juridical framework to say that while it's indeed doubtful that individual workers have been treated unjustly, the class composed of those individuals has been treated unjustly. But this is exactly what Marx is saying in chs 23 and 24 of Capital I. Workers suffer injustice only to the extent they identify as a class.
But how indeed can a class suffer an injustice if its members have not? How can Fruit be tasty if the concrete fruits do not taste good? What kind of crazy ontology is this? And so what if the working class is treated unjustly if workers as individuals are not treated unjustly?
But taking seriously the case for non justice..
We exchange the fruits of our own labor for the the fruits of others' labor. Strictly speaking the labor times need not be equivalent for the exchange to be just as long as the exchange is voluntary and serves the respective interests of the parties.
But it is indeed an injustice to exchange someone's else labor for even more of his labor, for this is not exchanging but appropriating. And certainly that can't be described as a just exchange even if we think what counts as justice is relative to the mode of production; understood scientifically or dialectically, this wage exchange is neither just...nor an exchange.
The injustice is not that the wage has cost the capitalist nothing (of course the money serving as variable capital does have an opportunity cost); it is that its cost has already been borne by the working class, so that the working class is having to buy back the fruits of its own labor with even more labor. If this is the content of the exchange, how can it be accepted as just even if equivalents are exchanged freely?
In Karl Marxs theory of Ideas, John Torrance suggests that wage transaction can be viewed as unjust iff the working class constitutes itself as a class and speaks in terms of interclass injustice. So if the working class sings together Solidarity Forever....
To charge injustice however the working class must organize itself and develop a theory of justice at odds with the juridic one in conformity with this mode of production. The latter task may be an unnecessarily complicated and time consuming as working class struggle can already be justified in terms of and is in fact actually pursued for non moral goods; and the struggle is also advanced by theoretical analysis of the consequences of capitalist production--that is, in terms of a partisan and scientific analysis.
The need for a moral or ethical theory or philosophy as presently understood is dubious. Again Marx is miles away from Nozick and Rawls.
Now to mark my difference with James D: I think the case for injustice depends on our taking not the point of view of what happens in exchange and production rather than just exchange. The case depends on our taking the scientifically more sound point of view of reproduction and social classes. Only from the dynamic and macrosocial point of view does the injustice become visible. If nothing in biology makes sense except in light of evolution, nothing in the science of capitalism makes sense except in the light of reproduction or self constraining production, production constrained by the need it be resumed again. Only from this perspective can we understood the actual exploitation of the working class, its veiled slavery, the fact that it comes over the course of economic reproduction to buy back the fruits of its own labor with a pledge of even more unpaid labor.
Again I think the problem--and I don't see how James D and Michael D are speaking to it-- is that rechtsbegriffe can't be used by the subject who has been treated unjustly in this way. That unjustly treated subject is not an individuated individual as the individual worker may not be buying back his own labor with even more labor of his own.
The only person who has suggested this argument is John Torrance Karl Marx's Theory of Ideas, but I think it's the crux of the matter. Nancy Holmstrom may have made the same point. I don't think Wood actually attends to this point.
Our conception of justice is relative to the mode of production, as Wood argues (though the argument was made first by Pashukanis who was ignored in the American debate except apparently by Jeffrey Reiman, How the Rich Get Richer, How the Poor Get Prison); our conception of justice is juridic arising out of the conflicts among individuals in their exchanges of commodities and money. Justice is simply an idealization of exchange--that it should be equal and free of fraud. So even if capitalism can be shown to be just, it still does not follow that it is transhistorically just as Elster puts it. But I don't think Marx was interested in condemning capitalism on the basis of a concept of justice foreign to this mode of production. That just doesn't fit with his critique of utopian socialism.
Rakesh