NOTES for Deu 23:23
Let us try at first to understand this sentence. Somebody could intend something of his assets for a freewill offering, which is a sacrifice to God. It didn’t mean that he had to go immediately to the closest altar and bring the promised thing, but that, what was so consecrated to God, became kadosh from the moment of the announcement. The lamb could run again months in the herd, but it was no more belonging to its former owner, although this one continues to assume the responsibility for it. This lamb could no more be eaten or sold; it was only possible at a certain moment to sacrifice it indeed. A decision said before God shouldn’t/can’t/ must not change!
The same principle is transferred on any vow made to God (see verse 22), but this verse can be interpreted again a little more widely: all that is said or promised, one way or another is said or promised before God, independently of the fact, that man swore or not, that is why it needs to be fulfilled in the same way as the promises of sacrifice are fulfilled. No " I have changed my mind " is accepted! Jesus will speak about the same thing later: "let your communication be, Yea, yea; Nay, nay": for whatsoever is more than these cometh of evil." (Matt. 5:37).