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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The marriage contract is valid once the offer of the Wali (legal guardian) - or his representative - and the acceptance of the husband occur in the presence of two witnesses, even if the bride’s guardian did not sign the marriage contract. What matters is the verbal utterance of mutual consent (the offer and the acceptance) and not the signature on the marriage contract; please refer to Fatwa 83629.
If the woman has a mental illness, such as insanity, and the husband was unaware of that until after concluding the marriage contract, then he is entitled to request annulment of the marriage. As to virginity, scholars held different opinions regarding the loss of virginity as a defect that legitimizes the annulment of the marriage.
For more information, please refer to Fataawa 81422 and 90593.
If it is confirmed that the husband in this case has the choice to annul the marriage after its consummation, then the wife is entitled to the whole Mahr (dowry); however, the husband is entitled to demand the Mahr from the person who deceived him. On the other hand, assuming that the husband does not have the choice to annul the marriage, then the husband is entitled to demand the difference between the Mahr of a non-virgin woman and that of a virgin according to opinion of some jurists.
If the woman denied being a non-virgin and swore to it, then she is entitled to her Mahr stated in the contract. In case of disputes over such issues, the case should be referred to a legitimate Islamic judge if available; otherwise, it should be referred to the scholars in the Islamic centers.
Allaah Knows best.