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 immediately to the nearest altar and bring what he had promised, but what was dedicated to God in this way became qadosh from the moment of declaration. A lamb could still run in the flock for months, but it no longer belonged to its former owner, although he continued to be responsible for it. That lamb could no longer be eaten or sold; at some point it could only actually be offered as a sacrifice. A decision spoken before God must not be revoked.

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