COMMENTARY on 12:66
A. Yusuf Ali:

Translation:
(Jacob) said: "Never will I send him with you until you swear a solemn oath to me, in Allah.s name, that you will be sure to bring him back to me unless you are yourselves hemmed in (and made powerless). 1728 And when they had sworn their solemn oath, he said: "Over all that we say, be Allah the Witness and Guardian!" 1729
Commentary:

1728  The appeal to the family's needs in the time of famine at length made Jacob relent, but he exacted a solemn promise from the brothers, under the most religious sanctions, that they would bring Benjamin back to him, unless they were themselves prevented, as the Insurance Policies say "by an act of God," so that they became really powerless. To that promise Jacob called Allah to witness.

1729  This is more than a formula. Allah is invoked as present and witnessing the bargain, and to Him both parties make over the affair to arrange and fulfil.