What is the legal basis for the permissibility of the marriage portion (Mahr)?

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The permissibility of the Mahr is established by the Book (Quran), the Sunnah, and consensus (Ijma'). The Quranic evidence includes the verse: "And it is lawful for you to seek [marriage] with them with your wealth, by taking them in marriage, not in fornication" (Quran 4:24). Further supporting evidence is found in the verse: "And give the women [in marriage] their Mahr as a free gift" (Quran 4:4). Abu Ubayd interprets this as meaning willingly, as the due prescribed by God. Another verse states: "So give them their due compensation as an ordinance" (Quran 4:24). The Sunnah provides evidence through the narration that the Prophet (peace be upon him) inquired about the Mahr paid by Abd al-Rahman ibn Awf upon his marriage, to which he replied he paid a weight of gold equal to a date-stone, and the Prophet blessed him and advised him to host a wedding even if only with a sheep. Furthermore, it is established that the Prophet (peace be upon him) freed Safiyyah and made her freedom her Mahr, agreed upon by both authorities. Muslims have reached a consensus on the legitimacy of the Mahr in marriage.