Meeraath (Inheritance) is not an act which has been assigned to the volition and discretion of man. The Muslim has absolutely no choice in the disposal of his assets after his demise. Even the one-third in which he is allowed to bequeath to non-heirs is by divine dispension. Allah Ta’aala, by His Volition, permitted the mu’min (believer) to act by his discretion in one third of the bounties (assets) in his possession.
Inheritance is a right which is confirmed for the heirs without the directive of the murith (testator) and without the volition and acceptance of the heirs. The heirs assume ownership of their shares simultaneously with the death of the murith. Regardless of the pleasure or displeasure of the testator or of his heirs, the latter become the owners of their respective shar’i shares. Even if an heir voices his disassociation from his share of the inheritance, he/she remains the owner of his/her respective share.
If a man for some reason says that he does not wish to be the heir of his murith, his wish is of no significance. His wish does not cancel his right of inheritance. He automatically becomes the owner of his share whether he accepts or rejects it. Example: A man in an argument with his murith, said: “If I accept anything from you by way of inheritance then my wife is divorced.” On the death of the murith, this person (the heir) automatically becomes the owner of his share of inheritance and talaaq comes into effect whether he accepts his share or not.
If a man refuses his share of inheritance, it is not transferred to the other heirs. The Qaadhi or ruler will deliver the heir’s share to him and compel him to accept. If necessary, the Qaadhi will order forceful entry into his home to deliver his share of the estate. Once the wealth gained by inheritance has been placed in the heir’s custody and possession, he/she may do with it as he/she pleases.
It should now be clear that every will, testament or codicil drawn up in conflict with the shari’ah is haraam and not valid.
Taken from Kitaabul Meeraath by Majlisul Ulama of South Africa