Parashas Vayikra · Aliyah 1 · Passuk 4
Chabad.org — English translation + Rashi
Torah Temima points to Megillah 8a (verified; alav distinguishes neder from nedava regarding responsibility if the animal is lost).
E-Daf.com — Tsurras Hadaf (size=3)
HebrewBooks / Shas PDF of the daf
English: It was taught: Rabbi Shimon says: “And it shall be accepted for him to atone for him” — that which is upon him, he is responsible for; that which is not upon him, he is not responsible for. From here: vows (neder) carry responsibility [if the animal is lost], and free-will offerings (nedava) do not.
R’ Eliezer #2 (מיעוט) / force of alav — “upon him” limits responsibility to an obligation that rests on the person (neder), not on a designated animal alone (nedava).
If one vowed “I am obligated to bring an olah” (neder) and the animal is lost, he must replace it. If he said “this animal is an olah” (nedava) and it is lost, he has no further obligation.