Is there any sin on the mother or the son if the mother gifts her dutiful son money during her lifetime, and makes the son sign a waiver of his inheritance share to ensure fair distribution among her undutiful children after her death?

The preponderant opinion is that it is obligatory for both parents to be fair in distributing gifts and endowments among their children, unless there is a special need that necessitates preference. It is not permissible to prefer one child over another due to their righteousness or disobedience. Therefore, it is not permissible for the mother to favor one child over others in a gift; she must equalize between them or reclaim what she has given. It is not valid to ask her to waive her right to inheritance before her death.