But Engels actual contention is far dodgier than this. Engels actually posits the existence of a historical stage of "simple commodity production" in which value is said to be the determining instance of exchange **in a conscious way on the part of producers**:
"Hence, the peasant of the Middle Ages knew fairly accurately the labor-time required for the manufacture of the articles obtained by him in barter. The smith and the cartwright of the village worked under his eyes; likewise, the tailor and shoemaker — who in my youth still paid their visits to our Rhine peasants, one after another, turning home-made materials into shoes and clothing. The peasants, as well as the people from whom they bought, were themselves workers; the exchanged articles were each one's own products. What had they expended in making these products? Labor and labor alone: to replace tools, to produce raw material, and to process it, they spent nothing but their own labor-power; how then could they exchange these products of theirs for those of other laboring producers otherwise than in the ratio of labor expended on them?"
http://www.marxists.org/archive/marx/works/1894-c3/supp.htm#law
This is in **direct opposition** to Marx's own conception of value as a relation imposing itself "behind the backs of producers"!
Now here's Marx, arguing against the political economist John Gray (and by extension, against Engels):
"Since labour-time is the intrinsic measure of value, why use another extraneous standard as well? Why is exchange-value transformed into price? Why is the value of all commodities computed in terms of an exclusive commodity, which thus becomes the adequate expression of exchange-value, i.e., money? This was the problem which Gray had to solve. But instead of solving it, he assumed that commodities could be directly compared with one another as products of social labour. But they are only comparable as the things they are. Commodities are the direct products of isolated independent individual kinds of labour, and through their alienation in the course of individual exchange they must prove that they are general social labour, in other words, on the basis of commodity production, labour becomes social labour only as a result of the universal alienation of individual kinds of labour. But as Gray presupposes that the labour-time contained in commodities is
immediately social labour-time, he presupposes that it is communal labour-time or labour-time of directly associated individuals. In that case, it would indeed be impossible for a specific commodity, such as gold or silver, to confront other commodities as the incarnation of universal labour and exchange-value would not be turned into price; but neither would use-value be turned into exchange-value and the product into a commodity, and thus the very basis of bourgeois production would be abolished."
http://www.marxists.org/archive/marx/works/1859/critique-pol-economy/ch02b.htm
^^^^^^ CB: Engels, as he says, is talking about the "Middle Ages" and a holdover of it in his own lifetime , which is not "bourgeois production" that Marx is talking about.