Slavery had a variety of different effects on the American economy, from giving wealthy Southern landowners a free labor force to potentially restricting economic growth in the South, which relied heavily on slave-driven agriculture. Scholars have debated this issue for decades, and there is not a clear answer as to whether the system of slavery was a net good or bad for the nation's economy.
In many ways, slavery was an economic benefit to those who owned slaves, if not the nation as a whole. Slave owners did not have to pay the vast majority of their workforces, but ultimately, slave owners did pay for their slaves, even if that money did not go into the workers' pockets. Slave owners had to pay to purchase their slaves and also paid to feed, house, and clothe these involuntary workers, though the amount of money they paid to do this is likely much less than what would have been fair wages at the time. Clearly, this was not a very good economic arrangement for slaves, who were not paid for their work, but some scholars have argued that their economic situation might not have been much better if they were freed because the conditions of the American working poor at the time were so unfavorable.
The overall effect of slavery on the American economy is also arguable with various scholars identifying some positive and some negative elements of the practice. The South did not make the same technological and industrial advancements as the North until after slavery had been abolished, and some scholars consider this to have been an economic disadvantage for the nation as a whole. However, in the run-up to the Civil War, the South was producing a lot of raw materials to be used in manufacturing thanks to the hard work of the slaves who did the vast majority of the labor in producing crops such as cotton and tobacco.