The Will should observe Justice
The will should be fair, in that one designates a part of the inheritance to his relatives without committing injustice against his qualified inheritors and without extravagance or stinginess. It is recorded in the Sahihayn that Sa`d bin Abu Waqqas said, "O Allah's Messenger! I have some money and only a daughter inherits from me, should I will all my remaining property (to others)'' He said, "No.'' Sa`d said, "Then may I will half of it'' He said, "No.'' Sa`d said, "One-third'' He said, "Yes, one-third, yet even one-third is too much. It is better for you to leave your inheritors wealthy than to leave them poor, begging from others.'' Al-Bukhari mentioned in his Sahih that Ibn `Abbas said, "I recommend that people reduce the proportion of what they bequeath by will to a fourth (of the whole legacy) rather than a third, for Allah's Messenger said:
«الثُّلُثُ وَالثُّلُثُ كَثِير»
(One-third, yet even one-third is too much.)''
[فَمَن بَدَّلَهُ بَعْدَمَا سَمِعَهُ فَإِنَّمَآ إِثْمُهُ عَلَى الَّذِينَ يُبَدِّلُونَهُ إِنَّ اللَّهَ سَمِيعٌ عَلِيمٌ ]
(Then whoever changes it after hearing it, the sin shall be on those who make the change. Truly, Allah is All-Hearer, All-Knower.) means, whoever changed the will and testament or altered it by addition or deletion, including hiding the will as is obvious, then
[فَإِنَّمَآ إِثْمُهُ عَلَى الَّذِينَ يُبَدِّلُونَهُ]
(the sin shall be on those who make the change. )
Ibn `Abbas and others said, "The dead person's reward will be preserved for him by Allah, while the sin is acquired by those who change the will.''
[إِنَّ اللَّهَ سَمِيعٌ عَلِيمٌ]
(Truly, Allah is All-Hearer, All-Knower.) means, Allah knows what the dead person has bequeathed and what the beneficiaries (or others) have changed in the will.
[فَمَنْ خَافَ مِن مُّوصٍ جَنَفًا أَوْ إِثْمًا]
(But he who fears from a testator some unjust act or wrongdoing,)
Ibn `Abbas, Abu Al-`Aliyah, Mujahid, Ad-Dahhak, Ar-Rabi` bin Anas and As-Suddi said, "Error.'' These errors include such cases as when the inheritor indirectly acquires more than his fair share, such as by being allocated that a certain item mentioned in the legacy be sold to him. Or, the testator might include his daughter's son in the legacy to increase his daughter's share in the inheritance, and so forth. Such errors might occur out of the kindness of the heart without thinking about the consequences of these actions, or by sinful intention. In such cases, the executive of the will and testament is allowed to correct the errors and to replace the unjust items in the will with a better solution, so that both the Islamic law and what the dead person had wished for are respected and observed. This act would not constitute an alteration in the will and this is why Allah mentioned it specifically, so that it is excluded from the prohibition (that prohibits altering the will and testament) mentioned in the previous Ayah. And Allah knows best.