http://apologeticspress.org/AllegedDiscrepancies.aspx?article=748&b=Genesis
Ishmaelites or Midianites?
by Eric Lyons, M.Min.
While enjoying a meal and listening to their brother Joseph cry out from the pit into which they had cast him, the sons of Jacob (minus Reuben) noticed a group of merchants coming from Gilead. Rather than killing Joseph and concealing his body, the band of brothers chose to “ sell him to the Ishmaelites” (Genesis 37:27). The Ishmaelites, in turn, took Joseph down to Egypt and sold him to Potiphar, an officer of Pharaoh and captain of the guard (39:1). Skeptics charge that the author of Genesis erred when writing about the details of Joseph being sold into slavery. They insist that a clear contradiction exists because Genesis 37:36 says that “the Midianites” sold Joseph “in Egypt to Potiphar, an officer of Pharaoh and captain of the guard” (emp. added), whereas Genesis 39:1 indicates that Joseph was sold to Potiphar by the Ishmaelites.
The casual reader of the Bible might be troubled by the different names given in Genesis 37:36 and 39:1. After a thorough study of the Scriptures, however, one easily can see that the names “Ishmaelites” and “Midianites” are used interchangeably. The book of Judges records that after Gideon and his 300 mighty men defeated their enemy,
The men of Israel said to Gideon, Rule over us, both you and your son, and your grandson also; for you have delivered us from the hand of Midian.... Then Gideon said to them, I would like to make a request of you, that each of you would give me the earrings from his plunder. For they [those whom Gideon and his men had just conquered—EL] had gold earrings, because they were Ishmaelites…. Now the weight of the gold earrings that he requested was one thousand seven hundred shekels of gold, besides the crescent ornaments, pendants, and purple robes which were on the kings of Midian(Judges 8:22,24,26, emp. added).
After Gideon had delivered the Israelites from the hand of Midian, he requested the golden earrings that the Israelites had plundered. Plundered from whom? From those whom Gideon and the Israelites had just conquered. And who were they? Like Moses, in his inspired historical narrative concerning Joseph, the inspired writer of Judges referred to the people of Midian as Ishmaelites.
The Midianites and Ishmaelites mentioned in Genesis chapters 37 and 39 were the same group of traders. This is not a contradiction; nor is it proof that Genesis was written by different authors. As Keil and Delitzch concluded:
The different names given to the traders...do not show that the account has been drawn from different legends, but that these tribes were often confounded, from the fact that they resembled one another so closely, not only in their common descent from Abraham (Gen 16:15 and 25:2), but also in the similarity of their mode of life and their constant change of abode, that strangers could hardly distinguish them, especially when they appeared not as tribes but as Arabian merchants, such as they are here described as being (1996).
REFERENCES
Keil, C.F. and F. Delitzsch (1996), Keil and Delitzsch Commentary on the Old Testament (Electronic Database: Biblesoft), new updated edition.