(Nay, but your own selves have beguiled you into something. So patience is most fitting (for me).) Muhammad bin Ishaq said, "When they went back to Ya`qub and told him what happened, he did not believe them and thought that this was a repetition of what they did to Yusuf. So he said,
(Nay, but your own selves have beguiled you into something. So patience is most fitting (for me).) Some said that since this new development came after what they did before to Yusuf, they were given the same judgement to this later incident that was given to them when they did what they did to Yusuf. Therefore, Ya`qub's statement here is befitting,
Then, in verse 81, it was said: ارْجِعُوا إِلَىٰ أَبِيكُمْ , that is, the oldest brother said: I am going to stay here. You all go back to your father and tell him that his son has committed a theft, and that whatever we are saying is what we have seen with our own eyes, and that the stolen property was recovered from his baggage before us.
As for the last sentence of verse 81: وَمَا كُنَّا لِلْغَيْبِ حَافِظِينَ (and we could not guard against the unseen), it means that ` the pledge we had given to you to bring back Benyamin definitely was given in terms of outwardly visible circumstances. We did not know what we did not see and control - thus how could we know that he would steal and be arrested for it leaving us helpless in this matter.' The sentence could also mean that ` we did our best to keep Benyamin protected seeing that he does nothing which would put him in trouble. But, this effort of ours could be within the limits of our outwardly visible circumstances. That this thing would happen to him, in absence of our vigilance and knowledge, was something we did not know about.'
Among Joseph’s stepbrothers there was perhaps one brother who was different from the others. He was the same brother who had advised in the initial stage that Joseph should not be killed but pushed into a dry well, so that any caravan passing by could take him with it. That brother faced the same situation in Egypt and he separated himself from the others. His sense of honour prevented him from facing his father before whom he had already been proven guilty of losing a brother. He did not want him to think him guilty of losing yet another brother.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.