In this updated version, see here, István Mészáros lucidly rescues Jean-Paul Sartre's existentialist anthropology from the philosophical hermeneutic watershed of anachronistic individualism. The author critically examines evidence for a complementarity between Sartre's phenomonological ontology with historical-materialism, while paying particular attention to how Sartre's work is largely contributive to the Marxist Humanist attention to forms of social consciousness. What is stressed is that although Sartre rejects the 'dialectics of nature', this is not a total rejection of the dialectical method, since Sartre's attention to issues of morality are squarely placed in a historically-specific social context, specifically with respect to the parameters and social practices of capitalism, whereby, in similar fashion to Marx's concern with the dialectical contradictions between authentic human development and alienation, 'nothingness', or 'free will', is limited by the extent to which the actually existing physical world forces mankind to be subservient to constrained subjectivities, of which meaningful sense of self and dignity are lost in translation.
PS: Note that Mészáros is often considered part of Marxist Humanism, a school that emphasizes Marx's early writings, in particular his Economic and Philosophical Manuscripts. Mészáros most famous book is on Marx's theory of alienation here (and where he applies Marx's theory of alienation to reassessing the socialist alternative, and the conditions for its realization, is here). For the more directly relevant economic aspects of Marx's critique and reconstruction of the surplus approach one only needs to read his Theories of Surplus Value and, obviously, Capital (and the extent to which Piero Sraffa revived Marxist Economic Theory - see here & here ). Mind you, this is not to suggest an epistemological break in Marx's work, as authors like Louis Althusser have propounded. For debates on the supposed structural discontinuity, see here (subscription required) and here.