Can a wife request the return of a right she waived?

Question:
If a man married a second wife and the first wife refused to spend the night with him or sit with him, and then she agreed afterward, does he have to give her a share?

Answer:
It is obligatory for her.
Q: But does he have to make up for her, or is there no need to make up for it?
The Shaykh: It is obligatory for her to submit to the ruling of the Shari'ah and to accept Allah's division, whether it is two, three, or four wives. If she refuses, she no longer has a share. If she refuses, her right is waived. She is the one who waived her right; nothing is made up for her.
Q: If she gave away her day and then returned?
The Shaykh: She has the right to return. She can say: "I want my day, or divorce me," because she may not be able to endure it.