Tafsir of Yusuf 12:78

They said—when they observed the signs of Benjamin being taken, seeking to evoke compassion—"O Aziz! Indeed, he has a father who is a very old man,"—advanced in age, who can hardly bear to be parted from him, and he finds solace in him for his perished brother. It is said: they intended by "old man" one who is great in status and description. According to both interpretations, the point of the benefit lies therein; otherwise, the information that he has a father is already known from what preceded. "So take one of us in his place,"—as a substitute for him—"for we do not hold the same position in his sight regarding love and compassion."

"Indeed, we see you are of the doers of good"—to us, so complete your goodness, for blessing is only truly attained by its completion. Or, it means: it is your habit to do good in general, so act according to your habit and do not change it with us, for we are the most deserving of people for that. According to the first interpretation, the "good" is specific; according to the second, it is general. The sentence, in both interpretations, is a concluding parenthetical clause, according to what some meticulous scholars have adopted. Others have argued that if the intent of the "good" is specifically toward them, the sentence is an introductory explanation for what preceded, since taking a substitute is a form of goodness toward them. If the intent is that this general quality is your practice and habit, it serves to emphasize what preceded, and mentioning a general matter by way of a conclusion is more appropriate for that.