The reward for keeping a promise and the gravity of breaking it
Question

What is the reward for keeping a promise and the punishment for breaking a promise?.
Praise be to Allah.Firstly: 
The verses of the Holy Quran and the hadiths of the Prophet (blessings and peace of Allah be upon him) indicate that fulfilling promises and covenants is obligatory, and they point to abhorrence of the sin of the one who breaks them or fails to fulfil them. Failing to fulfil them may go so far as to constitute kufr, as happened to the Children of Israel and others when they broke the covenant with their Lord and abandoned their pledge to Allah to believe in Him and follow His Messengers. Allah, may He be exalted, says (interpretation of the meaning):
“And fulfil (every) covenant. Verily! the covenant, will be questioned about” [al-Isra’ 17:34]
“and fulfill the Covenant of Allah” [al-An ‘am 6:152]
And He says, praising His believing slaves (interpretation of the meaning):
“Those who fulfil the Covenant of Allâh and break not the Mîthâq (bond, treaty, covenant)”
[al-Ra ‘d 13:20]. 
Many texts of the Quran and Sunnah clearly indicate that it is obligatory to fulfil promises and covenants, and it is haraam to break them and act in a treacherous manner. All the verses that mention promises and covenants indicate that both explicitly and implicitly. 
The practice of the Prophet (blessings and peace of Allah be upon him) and his Companions is the clearest testimony to that and application thereof. 
Secondly: 
Allah has decreed for fulfilling promises great benefits in this world and in the Hereafter, in addition to the obvious benefits of creating a righteous and stable society. Among these benefits are the following:
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Fulfilling promises is one of the attributes of the pious according to the Book of Allah, and it one of the greatest means of attaining taqwa (piety, consciousness of Allah). Allah says (interpretation of the meaning):
“Yes, whoever fulfils his pledge and fears Allah much; verily, then Allah loves those who are Al-Muttaqûn (the pious)”
[Aal ‘Imraan 3:76]. 
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Fulfilling promises is a means of attaining security in this world and preventing bloodshed, and of protecting the rights of people, both Muslims and kaafirs, as Allah says (interpretation of the meaning):
“but if they seek your help in religion, it is your duty to help them except against a people with whom you have a treaty of mutual alliance, and Allah is the All-Seer of what you do”
[al-Anfaal 8:72]. 
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It is a means of expiating bad deeds and gaining admittance to Paradise, as we see in the words of Allah in Soorat al-Baqarah (interpretation of the meaning):
“and fulfill (your obligations to) My Covenant (with you) so that I fulfill (My Obligations to) your covenant (with Me)”
[al-Baqarah 2:40].
Ibn Jareer said: His covenant with them is that if they do that, He will admit them to Paradise. End quote. 
In Soorat al-Maa’idah, Allah, may He be glorified, states that He took a covenant from Bani Israa’eel, then He explained that covenant and stated the reward for fulfilling it, as He says (interpretation of the meaning):
“Verily, I will expiate your sins and admit you to Gardens under which rivers flow (in Paradise)”
[al-Maa’idah 5:12].
And there are other reports which are clear to anyone who ponders the Book of Allah and the Sunnah of the Messenger of Allah (blessings and peace of Allah be upon him), both his words and actions. 
There are many verses and hadiths about this topic; we advise you to refer to the books Riyadh al-Saaliheen by Imam al-Nawawi (may Allah have mercy on him), and al-Targheeb wal Tarheeb by Imam al-Mundhiri (may Allah have mercy on him). 
Thirdly: 
Betrayal or treachery is the opposite of trustworthiness and loyalty. If trustworthiness and loyalty are the qualities of faith and piety, then betrayal and treachery are the qualities of hypocrisy and evil --Allah forbid. 
It was narrated that ‘Abd-Allah ibn Amr (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “There are four characteristics, whoever has them all is a pure hypocrite: when he speaks he lies, when he makes a promise he breaks it, when he makes a covenant he betrays it, and when he disputes he resorts to obscene speech. Whoever has one of them has one of the characteristics of hypocrisy, until he gives it up.” 
Narrated by al-Bukhaari, 3178; Muslim, 58. 
It was narrated that ‘Ali ibn Abi Taalib (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever breaks the covenant of a Muslim, upon him be the curse of Allah, the angels and all the people, and Allah will not accept any obligatory or naafil act of worship from him.” 
Narrated by al-Bukhaari, 1870; Muslim, 1370. 
It was narrated from ‘Abd-Allah ibn ‘Umar (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said:
“Allaah will set up a banner for the betrayer on the Day of Resurrection, and it will be said: This is the betrayer of So and so.”
Narrated by al-Bukhaari, 6178; Muslim, 1735. 
We ask Allah to make us among those who fulfil promises and covenants and to grant us refuge from treachery and breaking of promises, and to help us to do good in word and deed… Praise be to Allah the Lord of the Worlds. 
See: al-‘Ahd wal Mithaaq fil Qur’an al-Kareem by Prof. Dr. Naasir Sulaymaan al-‘Umar.