He did with her what he vowed to do, and he had vowed to offer her up as a burnt offering. Seems pretty clear cut to me.
Of course there is a translation controversy that hinges around the "and" in the biblical passage. Maybe it is better translated as "or".
Judges 11:31: "whatever comes out of the doors of my house to meet me, when I return in peace from the people of Ammon, shall surely be the Lord's, and (or?) I will offer it up as a burnt offering"
Hmmmmmm..... for an inerrant book, there seems to be a lot of problems with its accuracy.