Q. How come there is no mention that Avraham Avinu, who kept all mitzvos, made a pidyon habben when Yitzchok was born?

A. Horav Shlomo Miller Shlit’a explained that Avraham Avinu was considered to be a Cohen as mentioned in a number of midroshim, see also Ohr Hachaim (14: 18).
Besides, the Rov mentioned that it is unlikely that the mitzva of pidyon habben applied, before yetzias Mitzraim, since the holiness of the first born was not determined yet, as the posuk (Shemos 13: 15) clearly states: “And it came to pass when Pharaoh was too stubborn to let us out, Hashem slew every firstborn in the land of Egypt, both the firstborn of man and the firstborn of beast. Therefore, I slaughter [for a sacrifice] all males that open the womb, and every firstborn of my sons I will redeem.”

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a