He said, "Should I entrust you with him except [under coercion] as I entrusted you with his brother before? But Allāh is the best guardian, and He is the most merciful of the merciful."
Yusuf's Brothers ask Ya`qub's Permission to send Their Brother Binyamin with Them to Egypt
Allah says that when they went back to their father,
قَالُواْ يأَبَانَا مُنِعَ مِنَّا الْكَيْلُ
(they said: "O our father! No more measure of grain shall we get...") `after this time, unless you send our brother Binyamin with us. So send him with us, and we shall get our measure and we shall certainly guard him.' Some scholars read this Ayah in a way that means, `and he shall get his ration.' They said,
وَإِنَّا لَهُ لَحَـفِظُونَ
(and truly, we will guard him.), `do not fear for his safety, for he will be returned back to you.' This is what they said to Ya`qub about their brother Yusuf,
Therefore, he said: فَاللَّـهُ خَيْرٌ حَافِظًا (Well, Allah is the best guardian - 64), that is, he had already seen the outcome of their guardianship earlier, now he was placing his trust in Allah Ta` ala alone as his guardian. Then, he added: وَهُوَ أَرْحَمُ الرَّاحِمِينَ (and He is the most merciful of all the merciful - 64), that is, only from Him, he could hope that He would look at his old age and the sorrows he was surrounded with and would not let more shocks shake him.
In short, Sayyidna Ya` qub (علیہ السلام) did not rely on apparent conditions and on the pledges given by his sons, but did agree to send his youngest son with his brothers solely because he had now placed his trust in Allah Ta` ala.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 64–65
Joseph probably felt that it was discourteous to take the price of the corn from his brothers, or he perhaps thought that a lack of money might prevent them from coming back again. Therefore, he instructed his men that whatever amount his brothers had paid towards the corn should be quietly put among their belongings, so that when they went home and opened their bundles, they would find it and come again with their brother, Benjamin. Jacob, on the one hand, expressed his lack of trust in his sons where Benjamin was concerned. On the other hand, he also said that neither they nor anybody else had any power. Whatever God wished was bound to happen. But this happening is caused by human hands, so that an evil person, by performing evil deeds, may lay bare the truth about himself, while the good person, by performing righteous deeds, may have his name listed among the righteous.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.