The European Council for Fatwa and Research states the following: “Signing a contract in any transaction renders the contract obligatory on both parties, and neither of them is entitled to revoke it unilaterally without the consent of the other party. This will oppose what Allah and His Messenger (peace and blessings be upon him) ordained and the texts of the Qur’an and Sunnah confirmed. Almighty Allah says: “O you who believe! Fulfill all obligations.” (Al-Ma’idah: 1) Allah also says: “And fulfill (every) covenant for (every) covenant will be inquired into.” (Al-Isra’: 34) He also says: “Fulfill the covenant of Allah when you have entered into it and break not your oaths after you have confirmed them; indeed you have made Allah your surety.” (An-Nahl: 91).
The Qur’an in several verses strongly blames those who treat pledges lightly and break them after their being concluded. The Prophet (peace and blessings be upon him) regarded breaking a pledge as a branch of hypocrisy and a one of the basic traits of a hypocrite. “Four (traits) would render the one who is characterized by them an absolute hypocrite, and whoever has one trait of them has one trait of hypocrisy till he gives it up.” Then he mentioned among them “If he gives a pledge he will break it.” (Reported by Al-Bukhari and Muslim on the authority of `Amr.)
The contract does not have to be a written one, for mutual verbal agreement is enough to establish it. But either party has the option to revoke the contract, as we think, if he finds a better alternative, if the two parties are still at the site where the contact has been made, as is illustrated in the authentic Hadith: “The seller and buyer have the option (to change their minds) before separating.” (Reported by Al-Bukhari and Muslim on the authority of Ibn `Umar). The Hadith gives the option of revoking a contract to a person who has rashly and unthinkingly made it.
Similarly, a person who feels that he has been badly cheated, may raise his case to an arbitrating party that may give the option of revoking (the contract) because of being cheated, if that party is convinced of that, according to the Hanbali School and others.
A Muslim can escape the problem of revoking the contract after its conclusion if he stipulates for himself the option to annul the contract within a specified number of days during which he can revoke the transaction. This was what the Prophet (peace and blessings be upon him) advised one of the Companions to do when he complained of being cheated in business. The Prophet (peace and blessings be upon him) said to him: “If you do business, say: No cheating!” (Reported by Al-Bukhari and Muslim). In collections of Hadith other than Al-Bukhari and Muslim, it is reported that the Prophet (peace and blessings be upon him) said, “And I have the option for three days,” and Muslims should commit themselves to their conditions.
Otherwise, a Muslim should respect his own word, which is one of the values that Islam calls for, so that dealings are stable and people’s lives are straight and settled. A poet said: I never say one day “yes” to follow it with “no” even if I would lose all property and children.
Islam, moreover, prohibits the Muslim to compete with his brother who is about to conclude a transaction with somebody else and make a higher bid to snatch the transaction from him. In this respect the authentic Hadith states: “A Muslim should not compete with his brother for a (nearly concluded) transaction.” (Reported by Al-Bukhari and Muslim.)”