Marx says something interesting about the comparison between wage labor and slavery. Basically, that where the horrific nature of chattel slavery hides the way that the labor of slaves went toward their own subsistence, where wage labor hides the way that the labor of workers goes toward their employers:

In slave labour, even that part of the working-day in which the slave is only replacing the value of his own means of existence, in which, therefore, in fact, he works for himself alone, appears as labour for his master. All the slave’s labour appears as unpaid labour. In wage labour, on the contrary, even surplus-labour, or unpaid labour, appears as paid. There the property-relation conceals the labour of the slave for himself; here the money-relation conceals the unrequited labour of the wage labourer.

W.E.B. Dubois, in Black Reconstruction in America, says something similar about the state of chattel slavery prior to the civil war

The slavery of Negroes in the South was not usually a deliberately cruel and oppressive system. It did not mean systematic starvation or murder. On the other hand, it is just as difficult to conceive as quite true the idyllic picture of a patriarchal state with cultured and humane masters under whom slaves were as children, guided and trained in work and play, given even such mental training as was for their good, and for the well-being of the surrounding world.

The victims of Southern slavery were often happy; had usually adequate food for their health, and shelter sufficient for a mild climate. The Southerners could say with some justification that when the mass of their field hands were compared with the worst class of laborers in the slums of New York and Philadelphia, and the factory towns of New England, the black slaves were as well off and in some particulars better off. Slaves lived largely in the country where health conditions were better; they worked in the open air, and their hours were about the current hours for peasants throughout Europe. They received no formal education, and neither did the Irish peasant, the English factory-laborer, nor the German Bauer; and in contrast with these free white laborers, the Negroes were protected by a certain primitive sort of old-age pension, job insurance, and sickness insurance; that is, they must be supported in some fashion, when they were too old to work; they must have attention in sickness, for they represented invested capital; and they could never be among the unemployed.

To clarify, DuBois is not making a case for slavery, just describing the object conditions.

nothing is so calculated to ruin human nature as absolute power over human beings.

DuBois makes the strongest and most soaring case for the inherent dignity and personhood of Black Americans, and ultimately considers the chattel slaves who worked southern plantations as part of the working class, and analyses how these historic conditions were used by various kinds of capitalists, and the southern plantation owners were heavily subjected to market conditions. These conditions led to more heavy and brutal exploitation and abuse of victims of slavery as war and market conditions demanded more pressure for plantation owners to meet and compete with global market demands.

One of the main reasons that the wage labor of North America during and prior to the civil war was a material condition that led to the abolition of slavery, was because wage laborers were not paid to maintain themselves and their family, they were paid market rates which were heavily influenced by the way that slavery lowered wages for wage earning northern workers.

It was as Frederick Douglass said in Boston in 1865, that the Civil War was begun "in the interests of slavery on both sides. The South was fighting to take slavery out of the Union, and the North fighting to keep it in the Union; the South fighting to get it beyond the limits of the United States Constitution, and the North fighting for the old guarantees;-both despising the Negro, both insulting the Negro."

Furthermore, from DuBois:

Slowly but surely an economic dispute and a political test of strength took on the aspects of a great moral crusade.

The abolition of slavery was objectively a progressive development toward the dignity and humanity of the working class. Chattel slavery, which was (is) a form of exploited labor under capitalism, where it existed (exists) doesn't just harm the victim of slavery, but the waged worker who is forced to compete with free labor; and the efficiency of exploited waged work leads to more brutal exploitation of the victim of slavery.

But this division of labor is sustained in our current society as a difference between poor workers and middle class workers, still defined most often along lines of race, gender, and hegemonic social legitimacy. As such, the division of labor is a condition that suppresses the wages and wellbeing of all waged and unwaged workers.