That is something with the connotations that the word "slave" has in English, which are quite negative.
In Islamic culture, the term "slave" has different connotations.
For example, the grand vizier (=prime minister) of the Ottoman sultan was technically a slave of the sultan, and quite proud about that too. Same for most of his army generals and his civil bureaucrats. The sultan himself was always the son of a foreign slave girl. The sultan himself not a slave because his father wasn't.
A freeborn woman could not marry the sultan. Freeborn individuals were actually excluded from almost all central imperial positions of power, with the notable exception of the judiciary and the clergy. So, the entire bureaucratic elite that governed the empire were from top to bottom technically all "slaves".
Therefore, the Islamic notion of "slave" corresponds much more to "servant" in English, such as in the term "civil servant".
Furthermore, these "slaves" could often not get manumitted into freedom against their will. They could insist on lifelong slavery/employment, unlike freeborn people, who had no such right. You could divorce a freeborn wife but not a (pregnant) slave girl. Getting rid of slaves was actually an entire problem because the unwilling ones could even legally appeal against that.