Engels carefully monitored all reaction from economists and thinkers. Though illness consumed his body, his mind was still active. (Marx himself was 12 years dead.) During the last weeks of his life, he began his "Supplement to Capital, Volume Three".
On May 21, 1895, he wrote to Karl Kautsky:
"Meanwhile, I am about to send you a piece of work for _Neue Zeit_... supplements and addenda to _Capital_, Book III, No.1: Law of Value and Rate of Profit, reply to the doubts of Sombart and C.Schmidt. Later, No.2 will follow: the very considerably altered role of the Bourse since Marx wrote about it in 1865. To be continued according to demand and time available."
Engels had little time available -- he died August 5, 1895. He never progressed beyond a short draft of No.2, but No.1 would see print shortly after his death.
Engels endeavored to answer the parade of critics who sought to punch holes in _Capital_, vol.III. He begins with Marx's assertion that labor, not only theoretically but also historically, is the antecedent of the price of production; but this is altered with the historical development of exchange, which ultimately leads to simple commodity production evolving into capitalist production.
To quote the hard-copy introduction:
"Engels shows the law of value as the _law of motion_ of commodity production. He underlines the exceptional length of the epoch during which the law of value was effective. He traces the rise of capitalist relations in a number of actual historic examples and proves how these relations seize on the domain of production. Engels' essay is a splendid model of the genuine materialist explanation of the Marxian theory of value...."