Dr. Monzer Kahf, a prominent economist and counselor, states the following: “The Islamic distribution of the estate (that is given in the Qur’an, especially verses 10 and 11 of Chapter 4) is mandatory and a last will has no legal power, according to Shari’ah, to change, alter or modify it. Verses 12 and 13 of the same mention that “those” (meaning what is given above) are the Ordinances of God and promise ‘he who obeys” the Gardens of Heaven and “he who transgress His Ordinances” a humiliating punishment in the Fire of Hell. The definition of a Last Will in Shari’ah is restricted to disposition of one third only. Further, in application of Verses 12 and 13 of Chapter 4, the Prophet, pbuh, mentioned that “A last Will can’t give an heir.” so that the shares as prescribed in the Verses 10 and 11 and other verses too are not changed.
There is a unanimous, undisputed agreement among all Muslim scholars on this point. We in America and all other countries that have no Islamic inheritance system are required, from Shari’ah point of view, to write our last wills as a vehicle to implement the Islamic Inheritance system because a last will in all the 50 states’ laws is defined as a tool for distributing all the assets whereas the Shari’ah definition is only for distributing only a maximum of one third.
Consequently, it is gravely sinful for any Muslim who lives under inheritance laws that violate the Shari’ah to make a last will distribution that is different from the Shari’ah’s.”