Who was "the one" who opened his sack?
Rashi: Levi, who remained without a partner, 1 now that Shimon was incarcerated.
Hadar Zekenim: It was Levi, who was leading also Shimon's donkey. He would feed all the donkeys from Shimon's sack, 2 to lighten its load, for he needed to lead two donkeys.
Gur Aryeh: The Pasuk says "ha'Echad" with the definite article, rather than just "Echad." It must be Levi, who was left without his partner.
Perhaps it says "his sack" because he was responsible for it. (PF) Also refer to 42:27:2:2*.
Why did only one of the brothers need to open his sack to feed his donkey?
Ramban: Presumably, the brothers took with them hay for their donkeys, and one of their donkeys was weak and needed Mispo. 1 Consequently, only one of them opened his sack during the homeward journey; the others did not open theirs until they arrived home.
Moshav Zekenim: The one opened in order to give to all of their animals. Travelers do so. The next time, someone else would feed all from his sack. 2
Refer to 42:27:1:2.
Refer to 42:27:3:3.
Oznayim la'Torah: They all opened their sacks to feed their donkeys, but only Levi found his money. Yosef deliberately ordered his servant to place Levi's money in the sack of animal fodder, in order to punish him, 3 and the other brothers' money in the sacks of grain that they had purchased. Alternatively, Yosef ordered Levi's money to be placed at the top of his sack 4 and that of his brothers at the bottom.
Rashi (to Bereishis 24:25) says that "Mispo" refers to food for camels - straw and barley.
The verse says that he opened to give to his donkey! Perhaps he intended for his own, but once he opened, he would feed all of them. (PF)
Oznayim la'Torah: The Pasuk specifically states that it was found there.
What is the difference between 'Sak' and 'Amtachas'?
Rashi: They are one and the same.
Ramban citing Targum Onkelos #1: "Amtachas" means the load. In that case, each brother had one large sack and a few smaller ones to balance the load. Consequently, it was only Levi who happened to open the sack that contained their money. All the others, opened one of the other sacks during the homeward journey.
Ramban citing Onkelos #2: "Amtachas" is a large two-sided sack. 1 Levi happened to open the side into which the money had been placed. All the other opened the other side.
Malbim: Sak is the big sack for grain and fodder. Amtachas is a small Keli for special items. The money was put in the Amtachas at the bottom of the sack, and on top of it the grain and fodder. This is why they did not notice until they returned home, except for one, whom Chazal say was Levi. His Amtachas was at the top of his sack, and the money was at its opening, so he would notice and be pained also on the road, for he was more guilty for selling Yosef. Refer to 42:25:1:2 and the note there.
Ha'amek Davar: Most grain buyers put it in sacks. Yosef thought that also his brothers will do so. He said to put the money in the sack when they put the grain in. Yosef did not know that his brothers have Amtachos 2 (more esteemed Kelim) for grain, which go in the sacks, which are bigger. Fodder was put on the Amtachas, in the sack. When one took fodder from the sack, he saw his money in his Amtachas. Had he seen it in his sack, he would not have worried, for those who gave the grain engaged only with the Amtachos. Since it was in the Amtachas, he suspected that it is a plot to make accusations against them.
Called 'Matarasa' in Aramaic (Kesuvos 110a. Ramban - They are called Amtachas because they are stretched (Metach) to the sides). Perhaps he means that one sack was placed on one side of the donkey, and the other sack on the other side.
I.e. had he known, he would have commanded to put in the sacks, so they would not worry; he wanted to benefit them without pain. However, also according to Yosef's assumption, the brothers will see the money in their sacks, and worry that the grain givers placed it there! He could not command to put the money at the bottom of the sacks, lest the ones who gave the grain notice; it had to be at the opening, after they were full. (PF).
What is the significance of the fact that the money was found specifically in Levi's sack?
Malbim: Levi found it and was pained (lest he will be accused) also on the road, for he was more guilty for selling Yosef. Oznayim la'Torah - It transpires that each of the brothers received his just punishment. Shimon, who instigated throwing Yosef into the pit, was incarcerated. Levi, who assisted him, was the first to discover his money in his sack. The brothers, who subsequently sold Yosef, found their money only when they arrived home. Reuven, who began to save Yosef (and who was not involved in the sale), was spared from the troubles of responsibility for taking Binyamin down to Egypt and returning him, 1 and Yehudah, who was responsible for the sale, suffered in his place.
Oznayim la'Torah: When Yaakov refused his offer.
