Actually, the notion of indentured servitude for Africans is not as far-fetched as you might think. American slavery evolved slowly, with the notion that children of slaves were also slaves for live. The founding fathers (c. 1783) saw slaves getting freed (because the economy was lagging) and even George Washington did not think slavery had a long-term future.
My point: the atrocities with which we associate 19th century slavery evolved from 1810 to the 1840's, after the application of the cotton gin, as labor was needed for industrial cotton plantation work. Importation of slaves was no longer legal after 1808.
Actually, one way to mitigate the cruelty of slavery may have been to allow immigration of Africans under the conditions of indentured servitude, with the condition that the children were born free. Even the ancient Romans granted the rights of citizenship to the children of slaves.
Eventually, indentured servitude would be outlawed (maybe before 1865 as in OTL) and slavery wares itself out without a civil war.