Concerning the portions of inheritance assigned to each heir in the Holy Quran, Allah Almighty has said:
These are the limits set by Allah.(Surah an-Nisa’, 4:14)
Changing these portions according to our preference is not permitted (Surah an-Nisa’, 4:15).
It is not correct to write a will in favor of(Fiqh Ahmadiyya, vol. 2, p. 118)
a heir, except that a husband writes a will for the residence of his wife, in fact, writing such a will is necessary for him.