Why did the Ish see fit to mention that they moved away from there ("Nas'u mi'Zeh"), something which Yosef already knew?
Rashi: He was hinting to Yosef that they had veered away from the brotherhood. 1
Seforno: Even though he did not find them in Shechem (where his father had instructed him to go), he went beyond his father's instructions and went to look for them further.
Bereishis Rabah (84:14): They went away from Hashem's Midah. Chanukas ha'Torah (Vayeshev 39) - Hashem judges based on the present. They convicted Yosef based on the future (refer to 37:18:151:5).
Da'as Zekenim: They no longer wished to be twelve tribes (the Gematriya of "Zeh"). Ha'amek Davar - Why does it say "Ki"? You say that they are your brothers - [this is wrong]. They veered from brotherhood, for I heard... It was Hashgachah that Yosef did not make this inference. Ohr ha'Chayim - Yosef understood the man's words simply, for he thought that the Ish was a traveler. Had Yosef understood, he would not have put himself in danger!
Why did the 'man' say that 'he heard them saying... '? Why did he not just say that they were in Dosan?
Seforno: Because he did not actually see them going to Dosan. He only heard them saying that they would go there.
Rashi: The word "Dosan" is the acronym of 'Nichlei Dasos," which means wicked plans. He was in fact, saying that he overheard them 1 conspiring to kill him. 2
Ramban: He told Yosef innocently (as if he didn't know) that he had overheard shepherds discussing going to Dosan and that they may have been his brothers.
Targum Yonasan: He actually heard from 'behind the curtain' (directly from Hashem) that today the Shibud of Egypt had begun, and that Yosef's brothers moved to Dosan, following a prophecy that the Chivi were about to attack them.
Malbim: Chazal expounded that they went away from brotherhood, 3 for normally shepherds do not plan to travel to a far place. They thought that Yaakov will think that he was killed on the way from Shechem to Dosan. 4
Sifsei Chachamim: Otherwise, the word "Ki" ('because') does not go with his previous statement (refer to 37:17:1:1).
Here, Rashi says that according to the simple meaning, Dosan is the name of a place. In Sotah (13b DH mi'Shechem), Rashi says that it is not the name of a place; the Gemara said that Yosef was sold in Shechem (refer to 37:17:152). (His words could also be understood literally, which is how Yosef understood them; only in his capacity as an angel, they conveyed a deeper meaning. Otherwise, Yosef would have been foolish to have continued to look for them - Ramban.)
Rashi said so. I did not find an earlier source. Perhaps Malbim refers to Bereishis Rabah (84:14) (they went away from Hashem's Midah - PF).
This implies that that they already planned to kill him. Perhaps Reuven was waiting for the right time to try to dissuade them, or he had already protested (to no avail), just now (when they saw Yosef) he suggested not to kill him overtly. (PF)
Why does the Torah record that Yosef first went to Shechem, and then to Dosan?
Ohr ha'Chayim (to 37:14), Penei David (Vayigash 3, citing Derush Shmuel): This teaches why the merit of Yaakov's having escorted him 1 did not protect him (for Yaakov intended only that Yosef go to Shechem. 2 Ohr ha'Chayim - Yaakov saw with Ru'ach ha'Kodesh that he will not be harmed in Shechem. Yosef assumed that Yaakov was not adamant about Shechem; he merely told him where he expects that he will find them.)
Penei David: Mishchas Kodesh says that, due to the escort, he was saved from his brothers' intent to kill him. Perhaps this is like the opinion that Dosan is not the name of a place. Refer to 37:17:2:2**. (PF) Yerach ha'Eisanim says that the brothers went to Dosan in order that the escort will not protect Yosef!
Yosef found his brothers in Dosan, and they sold him there. But Sotah 13b says that since he was stolen from Shechem, his bones were returned to there (for burial, Yehoshua 24:32))!
Rashi (to Sotah 13b): Dosan is not the name of a place. It alludes to seeking 'Dasos v'Dinin' (justifications) to kill him. (Really, they were in Shechem. 1 )
Ha'amek Davar: The pit into which they threw him was between Shechem and Dosan. It was closer to Shechem, and within its Techum.
Imrei Emes (Shelach 5688): They already intended to do so in Shechem, so when they sold him in the end, it is attributed to their initial intent. 2
Gur Aryeh: Refer to 37:17:152.1:1.
If so, why must the Torah add that Yosef found them in Dosan? It says immediately after this that they wanted to kill him! (PF)
When they saw him, they first thought to kill him overtly. Reuven convinced them to cast him alive into a pit, and Yehudah suggested that it is better to sell him. This implies that they had not plotted to sell him beforehand! Perhaps intent to kill him 'includes' intent to steal him. (PF)
Why does it say that Yosef found his brothers in Dosan, and not that he went to Dosan and found them?
Ohr ha'Chayim: This shows that the "Ish" did not say absolutely that they are there, just he heard that they intend to go there.
Why does it say that Yosef went "Achar Echav"?
Ohr ha'Chayim #1: He did not understand that they left brotherhood.
Ohr ha'Chayim #2: Yosef understood that they left brotherhood, and he pursued brotherhood, to increase his reward - and he received it in Egypt.
QUESTIONS ON RASHI
Rashi writes: "They have departed from here - [I.e.] they have moved away from brotherhood." Why interpret this way, rather than the simple meaning?
Mizrachi, Gur Aryeh: Yosef had asked where they were grazing (37:16). The Ish need only have responded, "I heard them say, let us go to Dosan.'" This extra phrase is therefore open for interpretation.
Rashi writes: "'Neilecha Dosainah' (lit. let us go to Dosan) - [should be interpreted as] 'to search for Nichlei Dasos (treacherous judgements) against you.'" Why interpret this way?
Gur Aryeh: Once we say that "Nas'u mi'Zeh" means, "they have moved away from brotherhood;" we must interpret the following phrase, "for I heard them saying 'Neilecha Dosainah,'" as his support for this conclusion - "I heard them searching for ways to treacherously pass judgement against you, therefore I know that they are no longer to you as brothers."
Rashi writes: "In the simple sense, [Dosan] is the name of a place." But Rashi (to 37:14) writes that the brothers 'ruined their deeds' in Shechem, not in Dosan?
