In what system is it endorsing it in? modern or ancient? Universal or contextual? You are treating slavery very 1 dimensionally and culture the same way. Ancient cultures are not going to fit in our mold. The use of master/slave hierarchies are not innately evil and there may be a mixture of justice and injustice happening within these systems. When it comes down to it is how people are treated as well as how they are protected by the law of the land. We can't just label salvery the same way in all circumstances because we superimpose our image of what slavery is over it.
The American agriculture labour force has a lot of migrant illegals over worked and under paid who live in the US but do not have the rights of a citizen. Is this morally right or morally wrong? What would happen If they were all deported tomorrow and what impact do you think this would have in the agricultural sector or the cost of food in grocery stores? Negative or positive? What impact would it have on the migrant illegals lives and their families? Positive or negative?
Societies are complex organisms and non are fully egalitarian. I don't know the answers of the above agricultural labour force in America because it's complex and blanket statements don't work but it's a very marred system that needs attention. I do however know a great difference can be made by the way we treat people. We have hierarchies, classes and examples of modern salvery where there is a clear top and a clear bottom. We need to speak out to the social injustices happening and do not mistreat a person or devalue them because they are in a lower position.
So where does the bible fit in this and what value does it endorse when it comes to the treatment of slaves? How does that compare to surrounding cultures? Are these positive things or negative things?