The basic principle for the Muslim is to be honest and truthful, and he should not speak anything but the truth, because Allaah says (interpretation of the meaning):
“O you who believe! Fear Allaah, and be with those who are true (in words and deeds)”
And the Prophet (peace and blessings of Allaah be upon him) said: “I urge you to be truthful, for “Truthfulness leads to righteousness and righteousness leads to Paradise. A man will keep speaking the truth and striving to speak the truth until he will be recorded with Allaah as a siddeeq (speaker of the truth). Lying leads to immorality and immorality leads to Hellfire. A man will keep telling lies and striving to tell lies until he is recorded with Allaah as a liar.” Narrated by al-Bukhaari, 5743; Muslim, 2607, from the hadeeth of ‘Abd-Allaah ibn Mas’ood (may Allaah be pleased with him).
Reconciling between people is an important issue in Islam, one which brings a great deal of reward. There is also a stern warning against sewing discord between people. Because of the importance of reconciling people in the Muslim society and the seriousness of discord and conflict, Allaah has permitted lying in order to reconcile between people and to remove discord and conflict which may have negative consequences for the religious commitment of individuals and communities.
It was narrated from Abu’l-Darda’ (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “Shall I not tell you of something that is better than fasting, prayer and charity?” They said: Yes. He said: “Reconciling between two people, for discord between people is the shaver.” Narrated by al-Tirmidhi, who said: this is a hasan hadeeth. And it is narrated that the Prophet (peace and blessings of Allaah be upon him) said: “It is the shaver, and I do not say that it shaves hair, rather that it shaves religious commitment.” End quote.
Whomsoever Allaah honours and enables him to reconcile between Muslims, if he needs to tell lies in order to reconcile between them, there is no sin on him for that, and it is not permissible to describe him as a liar, because the matter is serious; it is the matter of shar’i interests which means that in this case, lying is permissible, as it is narrated in al-Saheehayn from the hadeeth of Umm Kalthoom bint ‘Uqbah (may Allaah be pleased with her) that she heard the Prophet (peace and blessings of Allaah be upon him) say: “He is not a liar who reconciles between people, conveying good messages and saying good things.” Narrated by al-Bukhaari, 2546; Muslim, 2605
As for swearing false oaths in order to reconcile between people, it seems that this is permissible.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) said:
What is prescribed for the believers is to use oaths sparingly, even if they are true, because swearing oaths a great deal may cause him to end up lying, and it is known that lying is haraam, and if a lie is accompanied by an oath, it is even more haraam. But if necessity or shar’i interests dictate that a false oath be sworn, there is no sin in that, because it is proven in the hadeeth of Umm Kalthoom bint ‘Uqbah ibn Abi Mu’eet (may Allaah be pleased with her) that the Prophet (peace and blessings of Allaah be upon him) said: “He is not a liar who reconciles between people, conveying good messages and saying good things.” She said: I did not hear him grant any concession concerning anything that the people say of lies except in three cases: reconciling between people, war, and what a man says to his wife, or a woman says to her husband. Narrate by Muslim in al-Saheeh.
If a person says in order to reconcile between people, “By Allaah, your companions want to reconcile, they want to come to an agreement, they want such and such,” then he goes to the others and says something similar, with good intentions and aiming to bring about a reconciliation, there is no sin in that, because of the hadeeth quoted above.
Similarly, if he sees someone who wants to kill another person unlawfully, or to wrong him in some other way, and he says, “By Allaah he is my brother,” in order to rid him of this wrongdoer who wants to kill him unlawfully or beat him unlawfully, because he knows that if he says that he is his brother, he will leave him alone out of respect for him, then he has to say such a thing in the interests of saving his brother from injustice and wrongdoing.
To conclude: the basic principle concerning false oaths is that they are haraam, except in cases where they will serve a greater purpose than lying, as in the three cases mentioned in the hadeeth quoted above.
Majmoo’ Fataawa al-Shaykh Ibn Baaz, 1/54.