I think we might be be defining terms differently. Given the rest of your comment, it seems you might have been giving it the narrower definition which only includes the unjust form (which in fairness to you is the most common usage these days). But what you quoted from me, even if we don’t call it slavery, is not per se unjust.
The most obvious example of a just acquisition of title to services (it is contrary to the natural law to have title to a person) is a voluntary exchange. If you have an employer, your employer has title to some services from you during a limited period of time. You voluntarily exchanged this title likely for payment of a certain wage and other benefits. Is this unjust? Of course not. Just as we can voluntarily exchange some reasonable services for a limited period time, so can we voluntarily exchange all the reasonable services we might provide for a lifetime–say, in exchange for a lifetime of food, shelter, clothing, medical care, etc. There’s nothing per se wrong with this (although it presents lots of opportunity for abuse, as history shows).
Some period of involuntary service (again, provided that service be reasonable, etc.) is also not always unjust, say, as punishment for a duly convicted crime (the US Constitution, for example, provides for this to this day).
I don’t think slavery as practiced in the US met the definition of just. Title was usually not acquired justly. Unreasonable services were required. People were treated like chattel (families separated, cruel animalistic practices like bits were employed, etc., etc.). As I mentioned in my prior post, experience in general showed the just form to have become merely theoretical, which is why the Church rightly advocated for its abolition everywhere (and why when we hear the word slavery today we have a good revulsion for it like you expressed, we don’t even think of just forms of lifetime servitude).
