In the Narrative of the Life of Frederick Douglass, Douglass describes the time when he was hired out by his master to work on Mr. Covey's farm. Mr. Covey works the slaves unmercifully and beats them when their bodies cannot handle the work. Douglass runs away in an attempt to make a plea to his master. However, his master sends him back to Mr. Covey. It is likely that his master does not want to lose money on the deal or to create a bad reputation with Mr. Covey and other plantation owners. In this situation, we see that slavery has had the effect on masters of viewing people as property and treating them accordingly.
When Douglass returns to Mr. Covey, they end up getting into a brawl because Douglass refuses to be beaten by him. Douglass overpowers Mr. Covey, but Covey does not admit to having lost the fight. Douglass resolves that he will be a slave in form but not in fact. Slavery has had the effect of institutionalizing the racial hierarchy that serves as a power structure in that society--although Covey has lost the fight, he will not admit to it and still upholds his position as master. Douglass has won the fight and can only resolve to view himself differently from the inside; on the outside, he must continue to perform his duties.
The section of the book that the question is referring to is chapters 9 and 10. In chapter 9, Douglass is with Master Thomas for 9 months. At the end of those 9 months, Thomas has decided that Douglass is too unruly to continue keeping around. Thomas decides to "lend" Douglass to Edward Covey for one year. That's when chapter 10 begins.
Covey has made a name for himself as one of the area's best slave breakers. Thomas believes that Covey can "train" Douglass to be a better slave.
Some slaveholders thought it not much loss to allow Mr. Covey to have their slaves one year, for the sake of the training to which they were subjected, without any other compensation.
Covey's methods of "training" aren't exactly kind. He basically resorts to merciless beatings of slaves until the slaves learn to do as they're told no matter what. Being more of a city slave, Douglass is unfamiliar with the work that Covey requires of him. Instead of teaching Douglass, Covey just beats him.
I lived with Mr. Covey one year. During the first six months of that year, scarce a week passed without his whipping me. I was seldom free from a sore back. My awkwardness was almost always his excuse for whipping me.
The beatings are so bad that after one of them Douglass escapes back to Master Thomas. Douglass is hopeful that Master Thomas will somehow give help in order to end the beatings. Master Thomas sends Douglass back because the original business deal that had been reached between Thomas and Covey said one year.
Master Thomas ridiculed the idea that there was any danger of Mr. Covey’s killing me, and said that he knew Mr. Covey; that he was a good man, and that he could not think of taking me from him; that, should he do so, he would lose the whole year’s wages; that I belonged to Mr. Covey for one year . . .
The conflict of these two chapters illustrates just how dehumanizing the institution of slavery is. Slaves are not people. They are property. As property, slaves like Douglass can be bought, sold, traded, and lent out like any other non-living material good. Covey doesn't think of Douglass (or any of his slaves) as people with feelings. If he did, he wouldn't beat them within inches of their lives. The fact that Thomas doesn't even consider taking back Douglass is important. Thomas can see that Douglass has been beaten to a pulp, but he refuses to back out of the business deal that had been arranged. Douglass is a commodity to Thomas, and Covey is supposed to be making Douglass a more valuable commodity.
I suppose the argument could be made that the Covey conflict doesn't necessarily equate slaves to non-living property. I think an equal case could be made that Covey (and other owners like him) view slaves as living; however, the slaves are not any different in concept than cattle or oxen that need to be whipped to get them to work. This doesn't change the fact that slave owners dehumanize slaves in order to exert their control.