using the word slave today ... might have had a different meaning than back in those times...

as i recall a slave in modern terms is one who does all your work around your house and is given a room and board and food... sounds like a house maid.

the term slave 1400 years ago... is more like the modern lower wage construction worker of today... because even slaves during the construction of the pyramids of egypt were paid... may it be they were paid just barely enough to survive but ... isnt that just minimum wage of today...

so the quran is probably trying say that no matter what social status you should still have some respect... i highly doubt its telling you to get you a slave...

just my opinion