That's not even the point. The point is, in the Old Testament, your god condones slavery and says it's okay. Back then, that wasn't a big deal, as you say, because slavery back then was a non-issue.
However, today, we know that slavery is wrong, was always wrong, and will always be wrong. It is morally repugnant to own a human being, regardless of what society says, and it always has been and always will be.
This means that your god, who is supposed to be the source of all morality, was either wrong back then (in saying that slavery was okay, when it's not) or changed his mind and thus changed the "moral compass" in our hearts to reflect that, which would still mean that at some point, he was wrong, which is impossible for your perfect god.
The only conclusion that can be drawn, therefore, is that, if your god exists, he is an immoral god, and if he doesn't exist, the people who wrote the bible were merely writing down their own morality in place of god's. It seems highly likely that ancient people would have no conception of the idea that slavery is morally wrong, and would therefore consider that their god would find it to be acceptable as well.
If god was perfect, and perfectly moral, and we get our morality from god, then he would never have allowed slavery in the past. (or genocide or rape or the murder of infants, all of which are commanded or condoned by god in the OT)
