Why does Yosef say, “I’ll do as you asked,” instead of simply doing it, i.e. taking the oath? The parsha should end off with something like, “Va’yaas kein,” or “Va’yasem yado tachas y’reicho” to swear, as Ya’akov had asked?
The Ohr haChaim answers that Yosef was telling his father that an oath was not necessary. “I will do as you asked,” even if you don’t hold my feet to the fire and make me swear to it.
The difficulty with this approach is that it begs the question of why Ya’akov insisted on the oath in the first place (surely he didn’t doubt that Yosef would carry out his wishes?) and how Yosef could second guess that decision. The Netziv suggests that Yosef may have thought that his taking an oath by “sim na yadcha tachas y’reichi” was disrespectful to his father (see also Targum Yonasan and Peirush Yonasan that elaborates on this idea), but Ya’akov was mocheil. The Kli Yakar sees a shakla v’terya here about the type of oath: Ya’akov initially insisted on nekitas cheifetz, but after Yosef’s protest, Ya’akov asked simply, “Hishava’a li,” for a less severe oath without nekitas cheiftez, which Yosef acquiesced to.
I think you can tease out another approach from the Ramban. Ramban suggests that Ya’akov wanted Yosef to take an oath lest the Egyptians balk at Ya’akov being buried elsewhere or at Yosef’s leaving to arrange the funeral. The oath gave Yosef ammunition to impress upon the Egyptians the seriousness of the matter. Perhaps we can read this idea between the lines of the exchange between Yosef and his father. Yosef, the palace insider, thought he would have the power to carry out his father’s wishes even without the oath in place. Ya’akov, however, perhaps already sensitive to the impending galus and the loss of stature his children would quickly suffer, saw things differently, and did not trust the favor Yosef found in Pharoah’s eyes to last. Therefore, he reiterated his insistence on the oath.
The Da’as Zekeinim quotes a Midrash that solves the problem a bit differently and agav urcha teaches a nice lesson. The Midrash interprets, “Anochi e’eseh ki’devarecha,” to mean that Yosef did as his father had done and asked Bnei Yisrael to pledge to move his bones to Eretz Yisrael for burial. It sounds like the Midrash is taking the words of the pasuk out of context and introducing this new idea of Yosef’s request, perhaps intending to reinforce the parallel between Yosef and Ya’akov, but I don’t think that’s really what’s going on. I think the Midrash also means to show that Yosef was trying to impress upon his father that, oath or not, he takes the commitment to bury Ya’akov in Eretz Yisrael seriously. What better way for Yosef to do that than by revealing that he will in time make the same request to move his own bones. To pledge or promise to blindly carry out the wishes of a parent is one thing; to demonstrate that you’ve absorbed the values behind that wish and applied them to your own life is even greater.
Like the Midrash, Ksav Sofer sees “ki'devarecha” as drawing a parallel between Yosef and Ya’akov, but with a little twist. Ya’akov’s insistence on administering an oath showed that he wanted to make every effort possible, to give 110%, to make sure that his request was taken care of. Yosef responded by pledging to act, “ki'devarecha,” as you, Ya’akov, acted - just like you put in 110%, so too, I will do the same to see your wishes fulfilled.
What the K.S. is saying is that it’s not the oath per se, that motivated Yosef. What motivated Yosef was witnessing the effort his father invested. While nothing is guaranteed, it makes sense that if you want to motivate your children to put their kochos into doing something, e.g. learning, tefilah, chessed, etc., you need to put your own 110%, your own kochos, into those same efforts.