Is it permissible for a woman's Mahr (dowry) to be her husband's quitting smoking, and is it permissible to conceal this agreement from people or to give a symbolic sum of money to register it officially while making the real Mahr the quitting of smoking?

The majority of jurists stipulated that the mahr (dowry) must be either money or a usufruct that can be rented, citing the noble verse: "And lawful for you are [all others] beyond these, [provided] that you seek them with your properties in marriage, not unlawful sexual intercourse" (An-Nisa/24). The Hanafis explicitly stated that the mahr is whatever is considered a valuable asset. In contrast, the Malikis, Shafi'is, and Hanbalis permitted a usufruct to be the sadaq (dowry), such as his service to her in work or teaching. Therefore, the four imams prohibit the abandonment of a sin from being the mahr.

It was reported from Umm Sulaym, may Allah be pleased with her, that she stipulated Islam as her mahr for Abu Talhah, and it is not money. Although this might be specific to Islam due to its immense benefit, it is advisable to agree upon a specific amount of money as mahr, and to stipulate that the husband abandons a sin (such as smoking) for the marriage to be concluded, while ensuring the sincerity of his repentance before proceeding with it.