Friedrich Engels in the 1885 preface to volume two of Capital:
On page 609 of the first volume (Das Kapital, 2nd ed.) we find the following quotation, “The possessors of surplus-produce or capital,” taken from a pamphlet entitled The Source and Remedy of the National Difficulties. A Letter to Lord John Russell, London, 1821. In this pamphlet of 40 pages, the importance of which should have been noted if only on account of the one expression “surplus-produce or capital,” and which Marx saved from falling into oblivion, we read the following statements:
“...whatever may be due to the capitalist” (from the standpoint of the capitalist) “he can only receive the surplus-labour of the labourer; for the labourer must live” (p. 23).
Marx makes the following comment (manuscript Zur Kritik, p. 852): “This little known pamphlet — published at a time when the ‘incredible cobbler’ MacCulloch began to be talked about — represents an essential advance over Ricardo."
Our pamphlet is but the farthest outpost of an entire literature which in the twenties turned the Ricardian theory of value and surplus-value against capitalist production in the interest of the proletariat, fought the bourgeoisie with its own weapons.
So how did Marx save this "farthest outpost of entire literature," this "essential advance over Ricardo" from "falling into oblivion"? Engels already told us. On page 609 of volume one of Das Kapital, Marx included a rather innocuous quote from the pamphlet in a sampling of similar quotes from other authors.
That's it. That's the rescue.
Whatever else Marx may have had to say about the pamphlet was buried in manuscripts that would remain unpublished during Engels's lifetime.
Why does this even matter? Because what Marx called his "absolute general law of capitalist accumulation" is indistinguishable from the "two things" that the author of the pamphlet allegedly "overlooked."
The cornerstone of Marx's critique of political economy was a (mostly friendly) critique of a critique of political economy. I say "mostly friendly" because it seems Marx overlooked exculpatory evidence against the charge that the pamphleteer overlooked those two things. But that story is for another time.