The existence of slavery is quite troubling. Whether in ancient times or Today, the notion of slavery is offensive, especially to those whose predecessors were subject to such a notion. Why and how has slavery come to exist in the first place?
There are varying degrees to slavery: a legal or economic system under which people are treated as property. The nation which uses a currency not their own is, by definition of the economic pecking order and payment of tribute, subjects of the entity issuing currency. The term 'subject'/'slave' is replaceable, differing only by degrees of subjectivity.
History teaches how one king conquers another king, wiping out the losing population, or keeping a certain amount as subjects/slaves. The new subjects/slaves are moved elsewhere or left at their respective location, now owing yearly tribute to the conquering king. In due time, these subjects may assimilate, realize citizenship, or continue to be a subject class.
Have you considered the manner in which those residing in prisons are used? They are put to work, earning a small percentage of what a 'free' person would earn working the same tasks. The prison system is a business, using cheap labor as a commodity. However, prisoners are not termed 'slaves' or 'subjects,' despite them losing their freedom. Interesting. The political systems of old and new are businesses, using cheap labor as a commodity, but these commodities are not labeled subjects/slaves.
Considering the thought that nothing exists without reason or purpose, ancient laws would turn an outlaw into a slave for reparations. Don't we see the legacy of these ancient laws Today?
Regarding personal freedom and identifying who is or isn't a slave; the property 'owner' pays property taxes/tribute or else risks losing title to that property.
So, who is really free or not a subject/slave?