Why did the manager ascribe the treasure that they found in their sacks to their G-d and the G-d of their fathers?
Rashi: What he meant was that it was in their merit or in the merit of their fathers.
Rashbam: He mentioned the G-d of their fathers because everybody knew that their family was accustomed to miracles.
Ramban #1 (citing the Ibn Ezra): It seems that someone had a 'treasure' in his house and forgot about it, and that their G-d had placed that treasure in their sacks, 1 because he had received their money.
Ramban #2: "Matmon" means (not a treasure, but) something that is hidden. 2
A tall order, as how could ten bundles of 'treasure' tally with the amounts that each of the brothers paid for the grain? (Ramban).
The merchants tended to leave the money for their purchase in their sacks which the man in charge of sales would remove before filling the sack with grain (Refer to 44:1:2:1*). What purportedly happened on this occasion (according to Menasheh's explanation) was that ten men left their sacks with the seller with the money inside, and which he had mistakenly filled without removing the money. Yosef then came, and, in his haste to send his brothers home, ordered Menashah to pour the contents of the ten sacks into the brothers' sacks with the money still inside. Needless to say, this was all part of the story that Menasheh told the brothers, but was not true.
He should have said that the money belongs to the owner of the grain, like it says in Bava Metzi'a (26b)!
Moshav Zekenim (20): We must say that Yosef bought from many merchants. 1
רש"י: ואם אין זכותכם כדאי וכו': מנין להוסיף זאת?
גור אריה: אם לא כן- למה הוסיף "ואלוקי אביכם".