Why did Menasheh change his stance in the middle, agreeing with the brothers' suggestion and then suggesting something else?
Rashi, Seforno and Rashbam: Strictly speaking, he told the brothers that they were right - If a theft is discovered among a group of ten men, they are pronounced guilty. 1 However, he would go beyond the letter of the law, taking only the culprit as a personal slave (instead of the death-penalty), and allowing the rest to go free. 2
Ramban #1: Based on the fact that, following Menasheh's accusation that they were all guilty of the theft, Yehudah drew a distinction between the sentences of Binyamin and the brothers, it was clear that they were not aware of the crime, and that their volunteering to become slaves was in the form of a penalty. And what Menasheh therefore meant was that, based on their explanation, that only Binyamin was guilty and they were innocent, he would take Binyamin alone as a slave, 3 and allow them to go free.
Ramban #2: Alternatively, he meant to say that, unlike their claim that they were innocent, and that they penalized themselves to become slaves, they were obligated by law to be incarcerated until it became clarified that they were indeed innocent. 4
Da'as Zekenim: Indeed, returning the money shows that you are trustworthy - but Binyamin (and Shimon - Hadar Zekenim) did not return anything. He (Hadar Zekenim - or Shimon) stole it without your knowledge! 5
Rosh, Moshav Zekenim: I will do like your words ("we are slaves") to the thief [and not kill him]. Hadar Zekenim - I will also be lenient and totally exempt the others.
Rashbam: As is customary among businessmen. Moreover, bearing in mind the fact that it was the goblet of the ruler who was so good to them in that he returned their money, they all deserved the death-penalty - Seforno).
Since, although they were all together when the goblet was found, they may well not have been involved - as they claimed, and what he wanted was payment for the theft, not the death-penalty. The Ramban refutes Rashi's explanations, because, according to Rashi, the words "Gam ... Kein hu" do not fit well (though he attempts to explain them in different ways).
As the Midrash specifically states.
If presumably, Binyamin stole it, why did he begin searching from the oldest? Rashi (verse 12) said that he did so lest it not seem that he knows who has it! (PF)
Why did Menasheh say "you will be a slave to me"? He should have said "you will be a slave to my master"!
Da'as Zekenim, Moshav Zekenim: [Menasheh] needed to pay for [loss of] all the Kelim over which he was appointed. (Therefore, it is as if the thief stole from him.)
רש"י: אמת כדבריכם: רמב"ן: אם אמת שיש להעניש את כולם, למה המשיך יוסף "חלילה לי"?
רא"ם: יוסף לא אמר 'חלילה לי מלקחת אתכם לעבדים' אלא 'חלילה לי מלחשוד בכם שנגבתם'. 1
גור אריה: האחים אמרו שהם יהיו עבדים אף על פי שהם אינם חייבים, ועל כך אמר יוסף "חלילה לי" לעשות כך על פי דבריכם, אבל מן הדין כולכם חייבים כי אחד מכם גנב, אלא שאני נוהג בכם לפנים משורת הדין.
הגור אריה מוסיף הסבר ברא"ם: אם נמצאת גניבה ביד עשרה כולם חייבים כי כולם חשודים שידעו, אבל אני אינני חושד בכם ולכן "חלילה לי".