What is the ruling on a wife's waiver of a quarter of the advanced dowry before consummation, and her verbal waiver of the deferred dowry later on, and does she have the right to retract her waiver and demand it again?

There is no blame upon the woman for what she did. Her waiver of part of her mahr (dowry) after the marriage contract is valid, whether it was before consummation or after, provided she is an adult, sane, and of sound judgment, and has the legal capacity to dispose of her own wealth according to the majority of jurists. This is based on the Almighty's saying: ﴿إِلَّا أَنْ يَعْفُونَ﴾ (unless they [the women] forgo it), and His saying: ﴿فَإِنْ طِبْنَ لَكُمْ عَنْ شَيْءٍ مِنْهُ نَفْساً فَكُلُوهُ هَنِيئاً مَرِيئاً﴾ (But if they, of their own good pleasure, remit any part of it to you, take it and enjoy it with good cheer). What she has waived from her mahr cannot be claimed by her afterward, because what has been relinquished does not return.