Jephthah came to Mizpah to his house; and, behold, his daughter came out to meet him with tambourines and with dances: and she was his only child; besides her he had neither son nor daughter.
(2) Jephthah was thoroughly familiar with the Book of the Law of Moses and the laws governing sacrifices.  In that Law, the first-born, who were required to be "offered" to Jehovah, were never offered as a burnt-offering, but they were "redeemed" by the offering of a lamb instead. This applied even to the first-born of a donkey!  It is simply inconceivable that Jephthah would have been ignorant of this principle, or that he would have failed to take advantage of it on behalf of his daughter.