Qur’an...
Qur’an: Whoever then alters it after he has heard it, the sin of it then is only upon those who alter it: The pronoun in “the sin of it” refers to the alteration; all the rest refer to the bequest according to usage; bequest is a masdar and is used both as masculine and feminine.
(That is why the pronouns used in the verse are of masculine gender.) “upon those who alter it”: Allah did not say 'upon them', because the wording used clearly indicates the nature of sin, that is, alteration of a will; also it paves the way for the next verse.
The verse branches out from the preceding one. Its meaning is as follows (and Allah knows better): The sin of alteration is only upon those who alter the bequest (which was made according to usage).
Of course, if someone fears that the bequest of the testator is, or shall be, an act of disobedience or inclined towards a wrong course, and then effects an agreement between the parties, free from error and sin, then there is no blame on him; because he has not changed a bequest made according to usage; rather he has altered only the wrong and sin found in it. Traditions Muhammad ibn Muslim says that he asked as-Sadiq (a.s.) whether a bequest was allowed in favour of an heir.
The Imam said: “Yes”. Then he recited this verse, “Bequest is prescribed... if he leave behind wealth, for parents and near relatives...” (al-'Ayyashi). This tradition is also narrated, with a slight change in wording, in al-Kafi and at-Tahdhib.
as- Sadiq (a.s.) narrates through his father from 'Ali (a.s.) that he said: “He who did not bequesth at the time of his death for those of his near relatives who were not his heirs, surely ended his deeds with disobedience.” (al-'Ayyashi) Imam as-Sadiq (a.s.) said about this verse: ”(It is) a right which Allah has given in the properties of men to the Master of this affair (i.e. to the Imam).” The narrator says: “I said: 'Is there a prescribed limit for it?' He said: 'Yes'.