NOTES for Deu 23:23

A person could designate something from his property as a voluntary offering, that is, a sacrifice to God. This did not mean that he had to go at once to the nearest altar and bring what he had promised; but what was dedicated to God in this way became kadosh from the moment it was declared. A lamb could keep running with the flock for months, but it no longer belonged to its former owner, though he still remained responsible for it. That lamb could no longer be eaten or sold; at some point it could only actually be offered in sacrifice. A decision spoken before God cannot be revoked!

The same principle is applied here to every vow made to God (see v. 21), but this verse can also be understood more broadly: everything that is said or promised is, one way or another, said or promised before God, whether or not a person swore an oath. Therefore it should be fulfilled in the same way promises of sacrifice are fulfilled. No "I changed my mind" is accepted. Jesus would later say the same thing: "Let what you say be simply 'Yes' or 'No'; anything more than this comes from evil" (Matt. 5:37).