The Marxian conception of Real Abstraction can be found all over the place; for example, in Simmel, Sohn-Rethel, Adorno, Toscano and more generally, scattered throughout critical theory.
Within the value-relation and the value expression included in it, the abstractly general accounts not as a property of the concrete, sensibly real; but on the contrary the sensibly-concrete counts as the mere form of appearance or definite form of realisation of the abstractly general … This inversion, by which the sensibly-concrete counts only as the form of appearance of the abstractly general and not, on the contrary, the abstractly general as property of the concrete, characterises the expression of value. At the same time, it makes understanding it difficult.
It is as if together with and besides lions, tigers, hares and all the other real animals, which as a group form the various genuses, species, subspecies, families etc of the animal kingdom, there also existed the Animal, the individual incarnation of the whole animal kingdom.
In the second edition of Capital, we find the famous phrase: '[t]he equalisation of the most different kinds of labour can be the result only of an abstraction from their inequalities, or of reducing them to their common denominator viz. expenditure of human labour power or human labour in the abstract', while in the French edition Marx added a comma, continuing '… and only exchange produces this reduction, by bringing the products of the most diverse kinds of labour into relation with each other on an equal footing'. 
For Marx the mystery of real abstraction lies in how the production process (labour) is concealed in the exchange between commodities through the medium of their universal equivalency in exchange value, this latter (materially performed, practiced) abstraction being at the centre of capitalist life. Far from being an idea that floats free of daily practice, abstraction here is concrete and real, having determinate effects in social relations. Abstraction here is not something thought; it is something done.
 The Philosophy of Money, p. 78
 Intellectual and Manual Labor, p. 69
 Introduction to Sociology, pp. 31-32
 Fanaticism, pp. 186-190
 'The Value Form', Das Kapital, pp.39-140
 Das Kapital 1st ed. p.234
 Le Capital I p.70