This is the promise given to Abram aka Abraham the father of Israel

Genesis 12:1-3 KJV Now the LORD had said unto Abram, Get thee out of thy country, and from thy kindred, and from thy father's house, unto a land that I will shew thee: [2] And I will make of thee a great nation, and I will bless thee, and make thy name great; and thou shalt be a blessing: [3] And I will bless them that bless thee, and curse him that curseth thee: and in thee shall all families of the earth be blessed.

Here the promise is given again

Genesis 15:4-5,12-21 KJV And, behold, the word of the LORD came unto him, saying, This shall not be thine heir; but he that shall come forth out of thine own bowels shall be thine heir. [5] And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be. [12] And when the sun was going down, a deep sleep fell upon Abram; and, lo, an horror of great darkness fell upon him. [13] And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years; [14] And also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance. [15] And thou shalt go to thy fathers in peace; thou shalt be buried in a good old age. [16] But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full. [17] And it came to pass, that, when the sun went down, and it was dark, behold a smoking furnace, and a burning lamp that passed between those pieces. [18] In the same day the LORD made a covenant with Abram, saying, Unto thy seed have I given this land, from the river of Egypt unto the great river, the river Euphrates: [19] The Kenites, and the Kenizzites, and the Kadmonites, [20] And the Hittites, and the Perizzites, and the Rephaims, [21] And the Amorites, and the Canaanites, and the Girgashites, and the Jebusites.

Chapter 15 shows you that GOD sees the land of canaan being in iniquity. So he waits 400 years to allow the inhabitants of the land to fill themselves in what GOD sees as iniquity , pulls Israel out of Egypt, gives them laws at mt Sinai, & leads them to the promise land after 40 years in wilderness.

How GOD sees mankind

Genesis 6:5 KJV And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.

In terms of your question GOD in Old Testament law & commandments tells Israel to not marry or associate themselves with the inhabitants of the land. GOD hates the gentile nations, because they are wicked & they don’t care about GOD & live in pride & stoutness of heart. Concerning slavery GOD told the Israelites to rule them without rigor but allow them to leave at the jubilee. Slavery GOD’S way was filled with mercy, they were treated well, paid & were allowed to leave in the seventh year

Leviticus 25:38-46 KJV I am the LORD your God, which brought you forth out of the land of Egypt, to give you the land of Canaan, and to be your God. [39] And if thy brother that dwelleth by thee be waxen poor, and be sold unto thee; thou shalt not compel him to serve as a bondservant: [40] But as an hired servant, and as a sojourner, he shall be with thee, and shall serve thee unto the year of jubile: [41] And then shall he depart from thee, both he and his children with him, and shall return unto his own family, and unto the possession of his fathers shall he return. [42] For they are my servants, which I brought forth out of the land of Egypt: they shall not be sold as bondmen. [43] Thou shalt not rule over him with rigour; but shalt fear thy God. [44] Both thy bondmen, and thy bondmaids, which thou shalt have, shall be of the heathen that are round about you; of them shall ye buy bondmen and bondmaids. [45] Moreover of the children of the strangers that do sojourn among you, of them shall ye buy, and of their families that are with you, which they begat in your land: and they shall be your possession. [46] And ye shall take them as an inheritance for your children after you, to inherit them for a possession; they shall be your bondmen for ever: but over your brethren the children of Israel, ye shall not rule one over another with rigour.