A Woman who Relinquished Rights to her Ketuba

Sarah relinquished her rights (mechila) to her ketuba in order to encourage her husband, Reuven, to give her a get. Now she wants to back out of this step. Perhaps we should say that simple oral mechila without a kinyan is not effective when the creditor (i.e., the wife) still has a document (i.e., the ketuba) that states that the debtor owes her money.