This answer has some examples you might find answer your question.

Something to consider: The act of freeing slaves in the South was, to put it lightly, controversial and expensive. The fear of a slave uprising was never far from the minds of slaveholders in places where slaves outnumbered whites, especially after Haitian revolution drove the point home that this could end poorly for them. To this end, in most of the South, freed slaves were legally required to leave the state within a given timeframe (usually a year), and their former owner was legally required to support them. In some states, freed slaves could be captured and re-enslaved if they were found out of the direct supervision of their "chaperone." Of course, there were free blacks that stayed in the south-- but they were far from "safe."

In either case, there were taxes associated with manumission that made it prohibitively expensive, especially when taking into account the economic value of the slave in the first place. This couples with the fact that people in the business of selling slaves would likely have refused to sell to a known abolitionist for the purpose on principle in the first place-- assuming it stopped there. Slavery was a central part of life in the South, and working to subvert it or white supremacy was famously dangerous, even well after the Civil War. Even if the slave was freed, they were still explicitly excluded from the protection of the law and the benefits of citizenship.

There was the Colonization movement, which ultimately founded Liberia by sending freed slaves there (Which is a whole other post in and of itself). To that end, there were several attempts to buy slaves for that purpose, but the only real significant progress happened as a result of the Civil War and wholesale emancipation.

Sources that didn't fit in-text:
"The Field of Blood", Joanne B. Freeman
"A Disease of the Public Mind", Thomas Fleming
"Confederate Reckoning", Stephanie McCurry