Is it Islamically permissible for the mahr (dower) to be the husband's performance of a religious duty instead of money, and must the mahr be specified in the marriage contract?

It is not permissible for the mahr (dowry) to be the husband's fulfillment of his obligations towards Allah Almighty, without anything in return for you, because the mahr is a compensation (mu'awadah), and here there is no compensation.

It is not a condition for the mahr to be in cash; rather, it can be a benefit, such as building a house or teaching the Qur'an or Hadith. It is worth noting that the Hanafis, Hanbalis, and one opinion among the Malikis do not permit the mahr to be teaching something from the Qur'an.

It is not valid for the mahr to be the husband learning a specific surah from the Qur'an, or paying his zakat, or performing an expiation he owes, or similar matters where there is no exchange or مقابل (muqābalah), according to the view of most scholars.