Awake 75 7/22 states
"It has been contended that remarks made by Jesus Christ in the Sermon on the Mount forbid his followers to take oaths. Jesus stated: “Again you heard that it was said to those of ancient times, ‘You must not swear without performing, but you must pay your vows to Jehovah.’ However, I say to you: Do not swear at all, neither by heaven, because it is God’s throne; nor by earth, because it is the footstool of his feet; nor by Jerusalem, because it is the city of the great King. Nor by your head must you swear, because you cannot turn one hair white or black. Just let your word Yes mean Yes, your No, No; for what is in excess of these is from the wicked one.”—Matt. 5:33-37; compare James 5:12.
Did Jesus mean that it is wrong for his followers to take oaths of any kind? No, we cannot draw that conclusion, for various reasons. ..."
They then explain how in Hebraw law they took oaths (which is exactly what Jesus was saying and replacing with a new command) and use that to say it is okay. Then they explain it is a conscience matter, in some circumstances, for you to determine if it is an oath that is against God's will.
Therefore the GB have determined that it is appropriate for JWs to make resolutions or to promise certain monthly amounts. They also have special full time servants sign an oath to obey the GB. Obeying Jesus' words are optional.