Deuteronomy 15:12-18 underscores at least three laws about the practice of slavery:
1. It was to be VOLUNTARY.
The Law specifies, "If a fellow Hebrew, a man or a woman, sells himself.."
It was not to be a forced bond-labor. It had to be voluntary. When a person would be in so much debt that he couldn't pay it back anymore, he usually would prefer selling himself as a slave. In other cultures, such slavery would become permanent and for generations. But, the Bible didn't permit that, unless the servant voluntarily willed to remain with the master (16-17).
2. It was to be TEMPORARY.
It says that when one has served for 6 years, in the 7th year "you must let him go free."
Slavery couldn't spill over into the 7th year, which was the year of emancipation and rest. In that way, though a slave would not receive any hire wages for six years, except the food and basic necessities he needed, his debts would ALL BE PAID within just 6 years. Imagine that!
3. It was to be EMANCIPATORY.
On the 7th year, the slave was not only released; the Law says, "And when you release him, do not send him away empty-handed. Supply him liberally from your flock, your threshing floor and your winepress. Give to him as the LORD your God has blessed you."
The 7th year not only liberated the slave from ALL his debts, it was also the year of blessing for him. He could start his own business with all that he received from his master!
That is why David said "I know, O LORD, that your laws are righteous..." (Psalm 119:75).
In modern times as always, slavery is certainly an evil- in the Old Testament, it was only permitted as a lesser evil though with a seed of freedom implanted in it. However, today there are other more wicked ways in which people try to make slaves of people for life. When products are sold by using addictive mechanisms, when employees are forced into helpless subjection through manipulative methods, when banks and businesses force people into eternal and violent debts, these are more evil than the slavery that was present in olden days.
Bond-labor and forced bond-labor must be opposed; especially, bond-labor of kids: it is wickedness because the kids are innocent and not indebted to anyone: in fact, the world is responsible for the kids' welfare. But, at the same time we must also oppose mechanisms of society and economy that violate freedom through deception and intellectual violence.