All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.
If a wife has willingly given up the deferred portion of her Mahr, while being of sound mind, she has no right to claim it later on and the husband is not obliged to pay it to her. Al-Mudawwanah reads, “If someone to whom you owe a debt absolves you of paying it, then if you say: I have accepted, then this is considered payment of the debt, and the debt is waived.”
However, in case of dispute among the creditor and debtor regarding absolving the debtor, the Islamic judge in a Sharee'ah-court, if available, should judge between them; otherwise, the scholars at the Islamic centers should carry out that task.
Allaah Knows best.