What is the ruling on a suitor obliging his fiancée to secretly return the mahr (dowry) after marriage, with his claim that the agreement is permissible because nullifying the marriage was an available option for her, and what is the ruling on concealing this from her guardian?

Allah has made the dowry obligatory for women, and for a man to compel her to return it, or to stipulate its non-payment beforehand, is akin to stipulating no dowry in the marriage contract, which is a corrupt condition by scholarly consensus. Some scholars invalidate the marriage contract with such a condition, while others void the condition and obligate the dowry of the like (Mahr al-Mithl). A wife may waive her dowry with her full consent after it has become due, not under compulsion. He is not permitted to take anything from it unless they agree upon khul' (divorce initiated by the wife) with her consent. This condition is impermissible because it resembles stipulating no dowry and compels the woman to waive her right. The husband is advised to fear Allah and adhere to the Shari'ah. The wife should inform her guardian about this condition and should not accept it unless she is in extreme necessity, along with an agreement on a non-refundable portion and documenting the husband's receipt of the dowry to avoid any future disputes.