Nazir 61b - Ba'al Yachel for a Goy

Tosafos (bottom of page) writes that there isn't any issur of ba'al yachel for goyim that binds them to keep their word. However, Tosafos in Avoda Zara 5b writes that if a goy would pledge a korban that is missing a limb and therefore invalid, they will be positively obligated to donate a proper animal as the korban. The mishne l'melech (Laws of kings 10:7) asks, where in the world does tosafos derive that goyim are included in the issur of b'al yachel, hence obligated to keep their word; the issur of b'al yachel and the mitzvah of keeping one's word was said specifically to jews, not goyim. He concludes by citing our Tosafos in Nazir who write explicitly that goyim are not metzuveh to keep their word. The Avnei Miluim (1:2) answers this question by saying that although they are not obligated to keep their word, when they make a pledge to the beis hamikdash there is a monetary obligation to fulfill their pledge, just as a formal acquisition would create a monetary obligation - amira l'gavoha k'msira l'hedyot. Therefore, they may not be obligated to keep their word, but they are obligated to pay their debts. The pledge of a korban to the beis hamikdash creates a monetary debt.

I remember that when i learned this tosafos in avoda zara (i had not seen the avnei miluim), i thought that this tosafos was a proof that even in the absence of a formal issur or mitzvah, a human being is obligated to keep their word. This concept seems necessary to explain how swearing prior to matan torah was binding, since there is no Torah committing them to keep their oath. Furthermore, this would explain the very essence of kabalas hatorah - that they were 'mushbah v'omed m'har sinai' - what binds them to keep their shavua of accepting the torah? the torah itself; what binds them to keep the torah? their shavua! v'chozer chalila.... But if we assume that keeping an oath is part of the essence of the koach hadibur that was giving to the human being, it would explain all this, and also explain Tosafos in Avoda Zara that goyim are also obligated to keep their word.