Is a verbal agreement between spouses before marriage to deprive the wife of inheritance in exchange for a deferred dower Islamically valid, and is the wife entitled to claim her share of the inheritance after the husband's death and her retraction from the agreement?

Waiving an inheritance before it is due is void. The wife has her share of her husband's inheritance. As for the deferred mahr (dowry), she is entitled to a mahr al-mithl (a dowry customary for a woman of her status), not the stipulated and agreed-upon amount. This is because the condition within it is invalid; the husband took into consideration her waiving of the inheritance, which was not valid. This falls under the ruling of the jurists regarding the invalidity of a condition in a marriage contract, which necessitates reverting to a mahr al-mithl rather than the stipulated amount.