Joseph's request to Pharaoh hinges on a vow he cannot break. Targum Pseudo-Jonathan renders it simply. "My father made me swear, saying, Behold, I die, in the sepulchre which I have prepared for me in the land of Kenaan there shalt thou bury me. And now let me go up and bury my father, and I will return" (Genesis 50:5).
A Jewish oath is not a casual promise. Joseph had placed his hand under Jacob's thigh (Genesis 47:29) — an ancient gesture of solemn vow-taking associated with the covenant of circumcision. To break it would be to desecrate not only his father's wish but the covenant itself. Joseph tells Pharaoh, in effect: I must go. I am bound by a sacred oath.
Pharaoh understood the weight of sworn things. Egyptian religion was built on oaths to gods and kings. But the Targum is also telling its Jewish audience something about kibbud av — honoring one's father — which rises here almost to the level of a commandment. Jacob had prepared a tomb, had chosen a place, had bound his son. Joseph, at the height of his power, lays down his scepter for a week to carry his father home. The Aramaic closes with a reassurance to Pharaoh: "and I will return." Joseph is loyal to both fathers — his own, and the empire that raised him.