As I've mentioned before, I've read the entire Bible (more than once, actually) in English and Greek.
Wow... just wow...
You wrote: "Slavery according to the Bible is a sin, why cause it is breaking the laws of the land, why wasn't it a sin in the days of Moses?,, ..simply because the laws of the land allowed slavery". You are, in essence, asserting that the God of the Bible doesn't see slavery itself as wrong - it's just one of those things that he's indifferent about - and that it was OK since it was legal back then.
Do you really believe that what is legal is, necessarily, moral? Is adultery, which is legal, also moral?
The question isn't whether the laws of the land allowed slavery or not! From a moral standpoint, slavery is either wrong or it isn't. I assert that whether two thousand years ago, to hundred years ago or two days ago slavery was wrong. I assert that regardless of the prevailing law of the land at the time, slavery was, is and will be wrong, no matter what the law happens to say.
That's your opinion and you are entitled to it but anyone who has studied the origins of a nations and the history of war and how a nation has to assemble as one body to survive will disagree, you have to pick the lesser of 2 evil, incorporate slavery or run the risk of genocide.
So you have read the Bible, fair enough then you know God has commanded many evils upon man, so why are you surprised. The Bible follows 3 different approaches to one making a moral decision;...
1. The decision has to be necessary, this is above all, is it necessary to kill someone to protect your family, yes. Is it a sin?,... yes. Is it accepted by God, yes
2. The law of the land has to be followed no matter what, the Bible makes it clear. Is it a sin to work on the sabbath? yes, during the Babylonion exile the Israelites where forced to work on the sabbath, was it accepted by God,. yes.
3. Finally the commandments, which where summarized into only 2 in the New Testament, Love God and Love your neighbor
You may find this conflicting but I don't cause the world doesn't turn smoothly you know and it doesn't always work to use one set of laws without any flexibility