If all heirs are in full agreement to voluntarily distribute the estate in different shares than what is specified in the Quran, can they do that?

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If the heirs agree voluntarily and without any compulsion amongst themselves to share the wealth equally amongst themselves, so that there will be no difference between males and females, this is permissible according to Islamic law as long as it is done voluntarily.
If one of the heirs voluntarily relinquishes their full or part of the inheritance shares and was fully mentally qualified to take such a decision, and that in their renunciation it is voluntary and not under compulsion, then their renunciation is valid and enforceable.