Question (Rashba): Why does he need to ask from a Beraisa? He should ask from the verse "Lo Sisov Nachalah"!

Answer (Ri): One might have thought that the verse is [merely] good counsel.

Explanation #1 (Rivam): Since it says "Roshei", which connotes [at least] two, and a Beis Din cannot have an even number of judges, we add one more.

Objection: Just the contrary! From "Roshei ha'Matos" we expound [that vows can be permitted through] a lone expert!

"Roshei" (the plural) refers to experts in general (any expert can permit alone), like the Rashbam explained. How can we expound from it that we require three?!

Explanation #2 (Ri): Without the verse, we know that since we require a lone expert, if so one commoner does not suffice, so we require [at least] two. A Beis Din cannot have an even number, so we add one more.

Question: Just the contrary! From here we can prove that She'elah applies to Hekdesh, for there (Nazir 31b), we establish Beis Shamai's words when he said "the black ox that left my house first is Hekdesh", and a white ox left, that this is when he said "had I known that a white ox left, I would not have said black, rather, white";

Beis Shamai say that it is Hekdesh, because his will and intent to be Makdish it are Mevatel his words;

Also here, when he asks about Hekdesh [to permit it], when he says "I was not Makdish with this intent", we should permit him, since Beis Shamai follow his intent!

Answer (R. Tam): Since Beis Shamai say that his intent is Mevatel his words, and [his intent] makes it Hekdesh, all the more so his words should be Mevatel his intent, that he says "I was not Makdish with this intent."

Explanation (Rashbam): Beis Hillel learn [this] from reasoning.

Question: Does R. Yochanan hold like Beis Shamai?! In Nedarim (77b), it connotes that he learns from "Zeh ha'Devar"!

Citation (77b - R. Yochanan): If a Chacham said an expression for a husband [to annul a vow], or a husband said an expression for a Chacham [to permit a vow], it has no effect, for a Beraisa teaches Zeh ha'Devar - a Chacham permits, and a husband does not permit.

Answer #1: According to R. Yochanan, we do not learn from the verse, rather, from reasoning;

He brings a proof from the verse only to teach that what he said has no effect, but not because he learns from Zeh ha'Devar.

Answer #2: He expounds Zeh ha'Devar, and even though he needs it for this, he expounds the Gezeirah Shavah, for there is no challenge to it.