Why do you think the slave-free north was much richer than the south, before the civil war?
If slavery makes a country rich, why isn't Africa rich?
Why wasn't Brazil, which imported ~4 million slaves (compare to ~200k for USA) significantly richer than the USA?
Native Americans had slaves:
> "An exhaustive search of some 725 late 18th/early 19th century ethnohistoric sources and 20th century ethnographic works indicates that predatory warfare, or preying on other groups for plunder and captives, was engaged in by virtually all Northwest Coast societies."
> "Source after source notes, either through specific instances or in general terms, and with almost monotonous iteration, that within this large area a prime motive for raiding was to gain captives for enslavement."
(From: https://www.jstor.org/stable/3773392?seq=1#page_scan_tab_con...)
Why weren't those people rich?
If you think slavery was instrumental to any rich nation getting rich, I suggest you read a lot more history, and look at who had the most slaves in history, and who has the most slaves today.