Is a wife entitled to return the mahr to her husband in exchange for divorce, given that the contract did not include a deferred mahr?

If the wife is harmed by not conceiving due to the husband's infertility or his unwillingness [to have children], she has the right to seek a divorce. She is not obligated to return the dowry to the husband unless it was agreed upon as consideration for the divorce, based on the hadith (definition) of the wife of Thabit ibn Qays. It is not a condition for the dowry to include a deferred part, and the deferred part is not forfeited by not being written in the contract.