The question becomes even more complex when we examine Ber. 43: 19-21:
"So they drew near the man who was over Joseph's house, and they spoke to him at the entrance of the house. And they said, "Please, my lord, we came down at first to purchase food. And it came to pass when we came to the lodging place that we opened our sacks, and behold! each man's money was in the mouth of his sack, and we returned it in our hands."
According to the OP's quote of Ber. 42:26-35, only one brother opened his sack in the hotel. The others emptied their sacks when they reached home and only discovered that each brother's money had been returned when they got home.
According to the way they later presented the facts to the one over Joseph's house, they claimed that they all noticed the returned funds at the inn; NOT when they reached home??
Rashi tells us (see Gen. 42:27) that the one brother who opened his sack in the hotel and discovered the money, was in fact Levi.
"The one opened: That was Levi, who was left alone, without Shimon, his companion." — [from Gen. Rabbah]
Medrash Bereishis Rabbah 70:7, also tells us that Yaakov Avinu designated Levi as the holy one of his children. (This was to fulfill giving Maaser (Tithe) on his children as well as his possessions.)
Therefore, Levi was the first Priest designated as "especially holy" in his family. He would be the one to pray and learn Torah all day, while his brothers would support him. (similar to how later generations gave Terumah and Maaser to the Priests and Levites)
If so, then when the brothers came down to Egypt, Levi did not bring money. His brothers took extra money which they gave to Levi, upon their arrival in the grain markets of Egypt.
IOW, when Levi picked out his purchases, the brothers would have simply handed Levi the amount needed, and then Levi paid with his brothers' donation.
The brothers themselves, had brought surplus travel funds, kept in their bags.
Now, when Joseph's Egyptian servants were sneaking money back into each pack, they put money into 9 bags that had other money inside; and they put money into Levi's bag which held no money inside.
So when Levi opened his bag, he saw money where there should have been none at all. So, he exclaimed that his cash was returned!
The other brothers at the hotel, were not phased that the same was done to their bags, because each of them took lots of surplus money with them on the trip just in case. So, when they also opened their bags to feed their animals, the sight of money did not surprise them, as they knew they were each carrying lots of change.
However, when they all reached home and "emptied out" their bags, they realized then that they each had too much money! So now they all knew that the money they saw back at the inn was in fact extra and had been returned as well (in addition to Levi's find).
Therefore, when they returned and spoke to the overseer of Joseph's home, they presented the story that "all of them" had found their money returned in the hotel, based on hindsight.
(They now knew that the money they saw at the hotel was in fact extra and when speaking about it now, referred to its sighting as occurring earlier in the hotel and not later at home.)
This now explains any contradictions or difficulties in the verses.
About ten years ago, I was discussing this question with Rabbi Lisker who lives near me in Kensington, Brooklyn, NY. He has a Shul and Kollel which we call "Lisker's".
Rabbi Lisker researched the issue and came up with the above "Chiddushei Torah" to explain the right answer.
I feel his chiddush has the ring of truth, so I like to say this over as a vort. This Shabbos alone I probably taught it about 7 times. :)
I hope this helps. :)