From the great discussions above, slavery in this country was always sin. It was human trafficking and as discussed above would earn the death penalty in the Old Testament. Even in the New Testament 1 Timothy 1:10 refers to menstealers along with whoremongers, liars, etc. Jubilee wouldn’t apply; it only applied to the legitimate and humanitarian forms of slavery mentioned above.
The result of this did not end with the Civil War. This country’s continuation of the sin of dehumanizing men and women of a specific race continued. Laws that gave ex-slaves full rights were stripped by those who refused to support or accept full equality of ex-slaves. We suffer the results of that sin today.
Therefore, any attempt to justify US slavery is no better than attempts to justify any other sin. I think moral relativism re any activity is just another attempt to deny the word of God. If the Bible has ruled on it, it is our instruction. God is not a respecter of persons no matter the importance of the people involved.
Many slave owners repented and set their slaves free before the Civil War. Repentance is the grace that God provides for any sin. Trying to justify a sin (by moral relativism) will leave us in the sin since we can’t justify ourselves.
Unfortunately, the church was not always true to the word of God but some were, especially Northern Methodists who preached against slavery as did their founder Wesley. That is in keeping with the church today wherein some separate over the Biblical position on gender identity. Thank God there was/is always a remnant of true believers.