COMMENTARY on 12:70
A. Yusuf Ali:

Translation:
At length when he had furnished them forth with provisions (suitable) for them, he put the drinking cup into his brother´s saddle-bag. Then shouted out a crier: "O you (in) the caravan! behold! you are thieves, without doubt!" 1737
Commentary:

1737  Joseph's plan was to play a practical joke on them, which would achieve two objects. Immediately it would put them into some consternation, but nothing comparable to what he had suffered at their hands. When the plan was unravelled, it would make them thoroughly ashamed of themselves, and dramatically bring home their guilt to them. Secondly, it would give him an excuse to detain Benjamin and bring their aged father into Egypt . He contrived that a valuable drinking cup should be concealed in Benjamin's saddlebag. When it was found after an ostentatious search, he would detain the supposed culprit, and attain his object, as the story relates further on.

 

Muhammad Asad:

Translation:
And [later,] when he had provided them with their provisions, he placed the [King's] drinking-cup in his brother's camel-pack. And [as they were leaving the city,] a herald 71 called out: "O you people of the caravan! Verily, you are thieves! 72
Commentary:
71  Lit., "an announcer" (mu'adhdhin) - a noun derived from the verbal form adhdhana ("he announced" or "proclaimed" or "called out publicly").
72  Commenting on this verse, Razi says: "Nowhere in the Qur'an is it stated that they made this accusation on Joseph's orders; the circumstantial evidence shows rather (al-aqrab ila zahir al-hal) that they did this of their own accord: for, when they had missed the drinking-cup, [these servants of Joseph remembered that] nobody had been near it [except the sons of Jacob], and so it occurred to them that it was they who had taken it." Analogous views are also advanced by Tabari and Zamakhshari in their comments on the last words of verse 76 below. This extremely plausible explanation contrasts sharply with the Biblical account of this incident (Genesis xliv), according to which the false accusation was part of an inexplicable "stratagem" devised by Joseph. If we discard-as we must-this part of the Biblical version, it is far more logical to assume that Joseph, who had been granted by the King full authority over all that belonged to the latter (see verse 56 above), had placed the royal cup as a present in the bag of his favourite brother; and that he did this secretly, without informing his servants, because he did not want anyone, least of all his ten half-brothers, to know his predilection for Benjamin. For a further explanation of this incident and of its ethical relevance within the context of Joseph's story, see note 77 below.