In fact, this happens when Crusoe's...
Robinson Crusoe has a very matter-of-fact attitude about slavery. His father warns him not to go to sea, admonishing him to stay home and enjoy the middle class life. He warns him that by becoming an adventurer, Crusoe leaves himself open to being enslaved.
In fact, this happens when Crusoe's ship is overtaken by a Turkish vessel. The entire crew is enslaved by the Turks. Crusoe is able to escape from this slavery, a situation he doesn't like, but learns nothing of a moral nature from what happened to him. As soon as his fortunes change, and he has the upper hand on a plantation in Brazil, he purchases a slave:
I bought me a negro slave, and an European servant also—I mean another besides that which the captain brought me from Lisbon.
Later, after he has been shipwrecked on his island for some time, native cannibals appear in canoes for a visit and his thoughts rove to slavery:
Besides, I fancied myself able to manage one, nay, two or three savages, if I had them, so as to make them entirely slaves to me, to do whatever I should direct them, and to prevent their being able at any time to do me any hurt.
When he saves Friday, he uses him as a quasi-slave.
It's useful to remember that in the late seventeenth century there was no abolition movement, and slavery was not cast in moral terms. Today, we see it as a fundamental and heinous moral violation of a person's human rights to enslave them. However, to Crusoe, it is just another thing that happens to him and that he does to others. It seems no worse than say, losing a comfortable job would in this day and age, and having to go to work at Walmart, or moonlighting somewhere to make ends meet while living in a rough circumstances: not at all pleasant, but simply the way life is sometimes. When Robinson later becomes the business owner, buying slaves is about the same as hiring minimum wage workers is in our world: the owner may realize it's not entirely a good situation for the workers, but it doesn't cause moral anguish.
Complicating all of this is race. As the first quote shows, Europeans are in the more privileged position of becoming indentured servants, meaning they will be set free at a certain time, whereas the "negro" is kept enslaved forever. Crusoe also almost immediately thinks of the natives he sees on his island as potential slaves. His attitude throughout the novel is highly racist. For example, he is glad that Friday has some European-like features (!), but it never occurs to him that Friday might be anything but his subordinate and inferior.
Crusoe doesn't cast slavery as a moral issue, but as a fact of life, and unthinkingly assumes that people of color are inferior and therefore fit to be enslaved. Attitudes have changed greatly in the last 400 years.
It is important to keep in mind that these are Crusoe's attitudes, not necessarily Defoe's: Defoe sometimes gives his characters a narrower moral range than he himself possesses, but in this case, Crusoe's attitudes seem to reflect what was common in his period.