But Marx says the surplus value does not rightfully belong to the worker. Now of course Marx says that there would be no justice in a transaction if one party was alone bearing the costs for both sides of the exchange. But that party is not a real individual at least in a bourgeois atomistic ontology; it is a social class, the working class. The working class is unjustly treated over the course of economic reproduction. But our ideal of justice is juridical; justice is, per historical materialism, only the ideal reflection of the juridical regulation of voluntary and fair commodity exchange among equal individuals. I don't know if this is true, but this seems to be Marx's argument.
At any rate, in the individual transaction with the worker, the worker may not bearing directly the costs of both sides of the exchange.
So how is he unjustly treated?
Because the ideal of justice has its roots in juridical practice which in turn is rooted in commodity exchange, justice will not lend itself to a condemnation of the free market exploitation of the working class. The working class will have to do without justice. This does seem to be Marx's argument. He is the enemy of ethical or neo Kantian socialism.
Of course an ethical nihilism may pave the way or rather has already paved the way for atrocity. So there is a most troubling problem here.
So I think if you want to construct an ethics, you'll have to work against Marx, the Nietzschean like nihilist. Or at least the ethics which are implied in Marx's critique is so deeply buried that its recovery will have to be much more than an interpretation of him. Perhaps that's what the author of what's wrong with exploitation is doing.
Rakesh