Muslims should set the best example in keeping their promises, even if it causes them to have to forego some interests.
Saleem ibn ‘Aamir relates: “There was a [provisional] treaty between Mu‘aawiyah, may Allah be pleased with him, and the Romans. However, he marched towards them in order to attack them as soon as the pact would expire. A man, riding a horse, came and said, ‘Allah is the Greatest, Allah is The Greatest! Let there be observance [of terms] and not treachery.’ They turned to look and found that it was ‘Amr ibn ‘Abasah, may Allah be pleased with him, who further said, ‘I heard the Prophet saying, ‘Whoever is in a covenant with people should not tie or untie a knot [i.e. take any action against them], until its time is over or he renounces the covenant on a mutual footing.’ Therefore, Mu‘aawiyah, may Allah be pleased with him, went back [without launching an attack].”

That was how the Companions, may Allah be pleased with him, were raised; they were taught that the consequences of treachery are grave, and so, they avoided it and always stood guard against it. Truly, we are in dire need of following their example and adopting their morals.
Indeed, treachery has wrought destruction on many people, limited their options and enclosed them in an unbreakable band of disgrace. It is known that a treacherous person’s affairs are never settled and Allah, The Almighty, does not accept from him or her anything, be it repentance or expiation. ‘Ali ibn Abi Taalib, may Allah be pleased with him, said, “We do not have anything except the Book of Allah and this sheet of paper about the Prophet . [..] He who breaks a covenant between him and another Muslim, upon him will be the Curse of Allah and that of the angels and all of mankind; from him, neither contrition nor ransom will be accepted.” [Al-Bukhari]

A treacherous person is vilified both in the worldly life and the Hereafter, when Allah, The Almighty, will expose him before all creatures. Abu Sa‘eed Al-Khudri, may Allah be pleased with him, narrates that the Prophet said: “On the Day of Judgment, there will be a banner raised for every person guilty of betrayal. It will be raised in proportion to the extent of his [or her] culpability; and there is no form of treason more serious than that committed by a ruler.” [Muslim]

Some scholars consider treachery a grave major sin. Why not, when Allah, The Almighty, Says in a Qudsi Hadeeth: “I will be the Opponent of three types of people on the Day of Resurrection: he [or she] who makes a covenant in My Name, but is unfaithful; he [or she] who sells a free person and devours his price; and the one who employs a laborer and derives his [or her] objective from him [or her], but does not compensate him [or her] for that work.” [Al-Bukhari]

In his guidelines to the army leaders, the Prophet would command: “Wage war in the Name of Allah and in the Cause of Allah. Destroy those who disbelieve in Allah. Fight and do not embezzle the spoils of war; do not break your pledge; do not mutilate [dead bodies] and do not kill children.” [Muslim]

Treachery is a trait that suggests meanness and debasement –worse than this –it is a characteristic of hypocrisy. The Prophet said: “He who has the following four [qualities] is a pure hypocrite and whoever has one of them has one characteristic of hypocrisy until he gives it up: if he is entrusted, he betrays [the trust]; whenever he speaks, he tells a lie; when he enters into a covenant, he proves to be treacherous; and when he quarrels, he behaves insolently." [Al-Bukhari and Muslim]

If we were to list bad morals, there would be nothing worse than betrayal that leads to hypocrisy and nothing more disgraceful than breaching a covenant.