Commento su Genesi 42:27

ויפתח האחד literally, AND THE ONE OPENED HIS SACK — This was Levi who remained alone (one) being without Simeon his companion (cf Targum Jonathan).

AND AS ONE OF THEM OPENED HIS SACK. One of them opened his sack in the inn to give therefrom some fodder to his ass, while the others did not open their sacks until they were with their father, just as it says, And it came to pass as they emptied their sacks,¹⁵⁵Verse 35 here. for perhaps the others took straw for their asses, lest they consume their entire loads on the way. But the one who opened the sack on the way had a weak ass, which required fodder, and he thus found the money in the mouth of his bag (‘amtachto’).
Now Rashi writes that amtachath is the sack, but Onkelos translated it as “load.” It appears from Onkelos’ opinion that each one in order to equalize his load, had large sacks and small sacks, and the total load of each one is called amtachath. Thus it happened that one of them found his money at the mouth of the particular sack he opened, while it did not happen to the others until they emptied all their sacks.
It seems plausible to me that amtachath is a large sack containing two sides, known in the language of the Talmud as matrata,¹⁵⁶Kethuboth 110 a: hafuchi matrata lamah li, meaning that if a person is carrying two leather sacks of equal weight on his right and left shoulders, why should he change one sack for the other? (See Rashi there.) and the money of each one was put at the mouth of one of the sides. Now it so happened that one of them opened the side where the money lay, but it did not happen to the others. This kind of sack is called amtachath because it stretches (yimtach) at the sides.

ויפתח האחד, one of them. According to a homiletic comment quoted by Rashi, the definitive article at the beginning of the word האחד is a hint that this was Levi, who, through his buddy Shimon having been detained by Joseph, was now “the one,” i.e. left alone without his closest companion.