Deal not unjustly, and you shall not be dealt with unjustly. [Al Qur’an 2:279]
O you who believe! When you deal with each other in transactions involving future obligations in a fixed period of time, reduce them to writing. Let him who incurs the liability dictate, but let him fear his Lord God, and not diminish aught of what he owes.”
So, Allah tells us that the most important rule when it comes to transactions is that it should be writen down in a contract. Allah speaks about loans in this ayah and by having a contract it will, Inshallah, allow us to avioid many of the disputes and conflicts that may occur between one another.