Marx, on the basis of his previous examination of constant and variable capital and surplus-value, draws the conclusion that “not every sum of money, or of value, is at pleasure transformable into capital. To effect this transformation, in fact, a certain minimum of money or of exchange-value must be presupposed in the hands of the individual possessor of money or commodities.” (Engels, Anti-Dühring, part one, chapter 12.)
Thus, by characterising the process [expropriation of the expropriators] as the negation of the negation, Marx does not intend to prove that the process was historically necessary. On the contrary: only after he has proved from history that in fact the process has partially already occurred, and partially must occur in the future, he in addition characterises it as a process which develops in accordance with a definite dialectical law. That is all. (Engels, Anti-Dühring, part one, chapter 13.)