This Ayah is the longest in the Glorious Qur'an. Imam Abu Ja`far bin Jarir recorded that Sa`id bin Al-Musayyib said that he was told that the Ayah most recently revealed from above the Throne -- the last Ayah to be revealed in the Qur'an -- was the Ayah about debts.
Allah's statement,
﴿يأَيُّهَا الَّذِينَ ءَامَنُواْ إِذَا تَدَايَنتُم بِدَيْنٍ إِلَى أَجَلٍ مُّسَمًّى فَاكْتُبُوهُ﴾
(O you who believe! When you contract a debt for a fixed period, write it down) directs Allah's believing servants to record their business transactions when their term is delayed, to preserve the terms and timing of these transactions, and the memory of witnesses, as mentioned at the end of the Ayah,
﴿ذَلِكُمْ أَقْسَطُ عِندَ اللَّهِ وَأَقْوَمُ لِلشَّهَـدَةِ وَأَدْنَى أَلاَّ تَرْتَابُواْ﴾
(that is more just with Allah; more solid as evidence, and more convenient to prevent doubts among yourselves.)
The Two Sahihs recorded that Ibn `Abbas said, "Allah's Messenger came to Al-Madinah, while the people were in the habit of paying in advance for fruits to be delivered within one or two years. The Messenger of Allah said,
«مَنْ أَسْلَفَ، فَلْيُسْلِفْ فِي كَيْلٍ مَعْلُومٍ، وَوَزْنٍ مَعْلُومٍ، إِلَى أَجَلٍ مَعْلُوم»
(Whoever pays money in advance (for dates to be delivered later) should pay it for known specified measure and weight (of the dates) for a specified date. )
Allah's statement,
﴿فَاكْتُبُوهُ﴾
(write it down) is a command from Him to record such transactions to endorse and preserve their terms. Ibn Jurayj said, "Whoever borrowed should write the terms, and whoever bought should have witnesses.'' Abu Sa`id, Ash-Sha`bi, Ar-Rabi` bin Anas, Al-Hasan, Ibn Jurayj and Ibn Zayd said that recording such transactions was necessary before, but was then abrogated by Allah's statement,
﴿فَإِنْ أَمِنَ بَعْضُكُم بَعْضًا فَلْيُؤَدِّ الَّذِى اؤْتُمِنَ أَمَـنَتَهُ﴾
(Then if one of you entrusts the other, let the one who is entrusted discharge his trust (faithfully).)
Allah's statement,
﴿وَلْيَكْتُب بَّيْنَكُم كَاتِبٌ بِالْعَدْلِ﴾
(Let a scribe write it down in justice between you) and in truth. Therefore, the scribe is not allowed to cheat any party of the contract and is to only record what the parties of the contract agreed to, without addition or deletion. Allah's statement,
﴿وَلاَ يَأْبَ كَاتِبٌ أَن يَكْتُبَ كَمَا عَلَّمَهُ اللَّهُ فَلْيَكْتُبْ﴾
(Let not the scribe refuse to write, as Allah has taught him, so let him write) means, "Those who know how to write should not refrain from writing transaction contracts when asked to do so.'' Further, let writing such contracts be a type of charity from the scribe for those who are not lettered, just as Allah taught him what he knew not. Therefore, let him write, just as the Hadith stated,
«إِنَّ مِنَ الصَّدَقَةِ أَنْ تُعِينَ صَانِعًا، أَوْ تَصْنَعَ لِأَخْرَق»
(It is a type of charity to help a worker and to do something for a feeble person.)
In another Hadith, the Prophet said,
«مَنْ كَتَمَ عِلْمًا يَعْلَمُهُ، أُلْجِمَ يَوْمَ الْقِيَامَةِ بِلِجَامٍ مِنْ نَار»
(Whoever kept knowledge to himself will be restrained by a bridle made of fire on the Day of Resurrection.) Mujahid and `Ata' said that if asked to do so, "The scribe is required to record.''
Allah's statement,
﴿وَلْيُمْلِلِ الَّذِى عَلَيْهِ الْحَقُّ وَلْيَتَّقِ اللَّهَ رَبَّهُ﴾
(Let him (the debtor) who incurs the liability dictate, and he must have Taqwa of Allah, his Lord) indicates that the debtor should dictate to the scribe what he owes, so let him fear Allah,
﴿وَلاَ يَبْخَسْ مِنْهُ شَيْئاً﴾
(And diminish not anything of what he owes,) meaning, not hide any portion of what he owes.
﴿فَإن كَانَ الَّذِى عَلَيْهِ الْحَقُّ سَفِيهًا﴾
(But if the debtor is of poor understanding) and is not allowed to decide on such matters, because he used to waste money, for instance,
﴿أَوْ ضَعِيفًا﴾
(Or weak), such as being too young or insane,
﴿أَوْ لاَ يَسْتَطِيعُ أَن يُمِلَّ هُوَ﴾
(Or is unable to dictate for himself) because of a disease, or ignorance about such matters,
﴿فَلْيُمْلِلْ وَلِيُّهُ بِالْعَدْلِ﴾
(then let his guardian dictate in justice.)