I say: The more persuasive opinion is that the marriage is valid in both cases, and an equivalent mahr is due. Allah knows best.
Section
If a legally competent woman says to her guardian, “Marry me off without a mahr,” and he contracts the marriage while either negating any mahr or remaining silent, the delegation (tafwīd) is valid; the same ruling applies when a master says to his slave-girl, “I have married you to him without a mahr.” Delegation is not valid for anyone other than a competent adult woman.
When a valid delegation has occurred, the dominant view is that nothing becomes due by the mere contract. Should intercourse take place, an equivalent mahr becomes obligatory, and—according to the sounder opinion—it is calculated on the basis of her circumstances at the time of the contract. Before intercourse she may demand that the husband determine a mahr and may withhold herself until he does so; according to the sounder view, she may likewise refuse to be handed over until the mahr so fixed is actually paid. Her satisfaction with what the husband fixes is required, though her knowing the precise amount of the equivalent mahr is not, according to the more apparent position. It is permissible, on the sounder view, to stipulate a deferred mahr, and even one that exceeds the equivalent mahr.
Another opinion holds that exceeding the equivalent mahr is impermissible when the mahr is of the same kind. If the husband refuses to fix a mahr, or if the spouses disagree about it, the judge shall assign her the current local cash mahr, payable immediately.
I say: The judge should assign an equivalent mahr, and the husband must be aware of its amount. Allah knows best.
According to the sounder opinion, it is invalid for an unrelated third party to assume payment of the mahr from his own property. A correctly imposed mahr has the same legal standing as a named one: if the husband divorces her before consummation, she is entitled to half of it. If he divorces her before both fixing a mahr and consummation, she is not entitled to that half. If either spouse dies before specification and consummation, the apparent view is that no equivalent mahr becomes due.
I say: The stronger position is that it does become due. Allah knows best.
Section
The equivalent mahr is whatever amount men ordinarily give for a woman similar to her. Its chief criterion is lineage: one begins with the closest female relatives whose lineage matches hers. The order is: a full sister, then a paternal sister, then daughters of a brother, then paternal aunts in the same sequence. If there are no agnatic women, or they have not been married, or their mahrs are unknown, one turns to uterine relatives such as grandmothers and maternal aunts. Age, soundness of mind, financial status, and virginity are likewise taken into consideration.