The ruling on swearing by God, whether truthfully or falsely

 

It is advised to say to him: “You should avoid excessive swearing, even if you are truthful,” based on the words of Allah, the Exalted: “And guard your oaths” [Al-Ma’idah: 89], and the saying of the Prophet May God bless him and grant him peace : “There are three whom Allah will not speak to, nor look at, nor purify on the Day of Resurrection, and they will have a painful punishment: an old man who commits adultery, a proud poor person, and a man who has made Allah his merchandise, not buying except with his oath and not selling except with his oath.”
The Arabs used to praise those who swore little.

It is recommended for a believer to minimize taking oaths, even if truthful, because frequent oaths might lead to lying. Lying is known to be forbidden, and it becomes even more severe when combined with an oath. However, if there is a necessity or a significant benefit, there is no harm in taking a false oath. 

This is based on the hadith of the Prophet May God bless him and grant him peace narrated by Umm Kulthum bint Uqbah bin Abi Mu’ayt (may Allah be pleased with her), where the Prophet May God bless him and grant him peace said: “He is not a liar who reconciles between people and says good or conveys good.” She added: “I did not hear him permit anything of what people say as lies except in three cases: reconciliation between people, war, and a man talking to his wife or a woman talking to her husband.” This hadith is recorded in Sahih Muslim.
So, if someone says during reconciliation, “By Allah, your friends love peace and want unity, and they desire such and such,” and then says the same to the other party with the intention of bringing about good and reconciliation, there is no harm in that due to the mentioned hadith.

Similarly, if a person sees someone intending to kill another unjustly or wrong them in some other way, and says, “By Allah, he is my brother,” to save him from the oppressor, knowing that if he says “my brother,” the oppressor will respect that and leave him, it is obligatory to do so to save his brother from injustice.

The principle is that false oaths are generally forbidden, except when they lead to a greater benefit that outweighs the lie, as in the three cases mentioned in the previous hadith.