In 37:28, the Pasuk states, "And the Midyanite traders passed by; they (Yosef's brother) drew Yosef from the pit and sold him to the Yishmaelim for twenty pieces of silver." Rashi states that those who drew him out were the sons of Yaakov, who then sold him to the Yishmaelim. The Yishmaelim then sold him to the Midyanim who, in turn, sold him to the Egyptians. Rashbam, however, says that while the brothers were eating a meal, the Midyanim came to the pit, took Yosef out, and then sold him to the Yishmaelim. According to this opinion, the brothers had no knowledge of the taking out and sale of Yosef, even though the Pasuk in 45:4 says, "I am Yosef your brother, whom you sold to Egypt," which would seem to say that the brothers had sold Yosef. Rather, this pasuk really means that the brothers caused Yosef's sale to the Egyptians by throwing him into the pit.
The Siftei Chachamim defends Rashi by saying that Midyanim could not have sold Yosef because he was not their's to sell in the first place. Therefore, they needed the brothers' permission to make this sale. This also disagrees with Rashbam's explanation of the Pasuk in 45:4 to say that the brothers really did sell Yosef.
The Da'at Zekeinim also agrees with Rashi and cites the previous pasuk in which the brothers state their intentions of selling Yosef. He also cites a Midrash that says that Yosef had been sold four times, but Rashbam only mentioned three groups who sold Yosef. He then poses a question as to why Midyan is spelled in two different ways. He also asks why the Rashbam seems to disregard the Pasuk which says that the Midyanites sold him to Potiphar. The Ibn Ezra answers the first question saying that the Midyanim and the Yishmaelim are the same people. He proves this from a Ramban which quotes a Pasuk from Shoftim 8:24, "The kings of Midyan, they are the Yishmaelim." From this we see that when the Torah mentioned the Midyanim in pasuk 37:35, it refers to the Pelishtim, and when it mentions the Midyanite merchants in pasuk 28, it is talking about a totally different nation.