Did Karl Marx write that slavery had good sides and abolishing it would indirectly destroy modern civilization?

Question

The WikiQuote page for Karl Marx claims he wrote this in a letter to Pavel Annenkov, 28 December 1846:

Freedom and slavery constitute an antagonism. There is no need for me to speak either of the good or of the bad aspects of freedom. As for slavery, there is no need for me to speak of its bad aspects. The only thing requiring explanation is the good side of slavery. I do not mean indirect slavery, the slavery of proletariat; I mean direct slavery, the slavery of the Blacks in Surinam, in Brazil, in the southern regions of North America.

Direct slavery is as much the pivot upon which our present-day industrialism turns as are machinery, credit, etc. Without slavery there would be no cotton, without cotton there would be no modern industry. It is slavery which has given value to the European colonies, it is the colonies which have created world trade, and world trade is the necessary condition for large-scale machine industry. Consequently, prior to the slave trade, the colonies sent very few products to the Old World, and did not noticeably change the face of the world. Slavery is therefore an economic category of paramount importance.

Without slavery, North America, the most progressive nation, would be transformed into a patriarchal country. Only wipe North America off the map and you will get anarchy, the complete decay of trade and modern civilisation. But to do away with slavery would be to wipe America off the map. Being an economic category, slavery has existed in all nations since the beginning of the world. All that modern nations have achieved is to disguise slavery at home and import it openly into the New World.

Did Marx write this?

Answer 1

tl;dr: No. Marx wrote that Proudhon would say that slavery had good sides (as part of his criticism of Proudhon).

This quote is from a letter from Marx to Annenkov. But it is important to look at the context to understand its meaning.

The sentence prior to the quote from OP is:

Let me now give you an example of Mr Proudhon's dialectics.

The final sentence that is removed from the quote is:

After these reflections on slavery, what will the good Mr Proudhon do? He will seek the synthesis of liberty and slavery, the true golden mean, in other words the balance between slavery and liberty.

It is clear that these aren't ideas Marx holds, but a rhetorical attribution of these ideas to Proudhon.

In The Poverty of Philosophy Marx lays out the same argument, and here it is again very clear that this is an attribution of these ideas to Proudhon:

For him, M. Proudhon, every economic category has two sides – one good, the other bad. He looks upon these categories as the petty bourgeois looks upon the great men of history: Napoleon was a great man; he did a lot of good; he also did a lot of harm. [...]

What would M. Proudhon do to save slavery? He would formulate the problem thus: preserve the good side of this economic category, eliminate the bad.

Marx's actual opinion of slavery can be seen in writing such as his letter to Lincoln in which he celebrates death to slavery, The Civil War in the United States in which he argues for the Union side and against slavery, or in Das Kapital where he argues that the workers movement is paralysed as long as slavery exists, and that there can be no labour emancipation without black emancipation.

Answer 2

No, he did not write that slavery had good sides and abolishing it would indirectly decay modern civilization.

Instead he wrote that someone else might think that slavery has good sides and abolishing it would indirectly decay modern civilization. There is no indication that he embraces this view or makes it his own. It becomes clear from the longer quote of the communication that was recently added at the linked location. In particular, if you dig a little deeper he seems to review a book in the letter where the quote in the question is taken from. He seems to be not agreeing with the ideas presented in the book, nevertheless repeats some of the positions of the author without endorsing them.

As others have already pointed out, the letter itself seems to be original material though.

I guess this question is a good example of how taking a too short quote of someone's work can lead to faulty conclusions.