Or HaChaim on Exodus 4:18/אור החיים על שמות ד׳:י״ח

1 אלכה נא, "I wish to go, etc." According to the agreement between Moses and Yitro reported in Shemot Rabbah 1,29, Yitro had made Moses swear an oath not to take Tzipporah away. Moses now had to ask Yitro to release him from that oath.

2ואראה העודם חיים, "and I want to see if they are still alive, etc." He did not tell Yitro a word about the mission G'd had asked him to undertake, seeing that G'd had not used the word לאמור when He had appointed Moses. We have repeatedly quoted Yuma 4 to the effect that one must not reveal information received privately unless authorised to do so.

3לך לשלום, "go and be well!" Yitro forgave him his oath. We learn from this that no formal annulment is necessary if the second party to the oath himself expresses his agreement that it be invalidated.