What did the brothers mean when they said, "Kaspeinu b'Mishkalo"?
Seforno: They meant that it was exactly the same coins that they had brought to pay for the grain that they found in their sacks, which led them to the conclusion that it was not a question of someone else's money having got mixed up with their own.
It was not true that when they arrived at the inn they all found their money at the top of their sacks. Only one of them did!
Oznayim la'Torah: After Yaakov accused them of divulging details of their families that Yosef had not asked them, they decided to leave out unnecessary details - unless asked.
Ha'amek Davar: They wanted to show that clearly, it was a ruse. This is not a lie, for [once one found, it] it was clear to them that just like his [money was at the opening of his Amtachas], also all of them.
They admitted that they found their money at the inn. How did they justify not returning it immediately?
Malbim: We could not return it immediately, lest our families starve. 1
One of some of them could have returned the money, and the other would bring the food to their family! Also, they delayed returning until they finished the food! Perhaps the delay was no longer than the additional travel time needed if they lived at the northeast corner of Eretz Yisrael. Why did they admit to finding the money before returning home? If each man's money was by the opening of his sack, clearly they would find it on the way. They and their donkeys must eat on the way! (PF)
Why did they mention that there was money at the opening of the Amtachas?
Malbim: That is where it is proper to be. Surely it was placed there intentionally!
