It should be noted that the mahr (dower) is the exclusive property of the wife, and she has full ownership and complete rights of disposal over it. Therefore, it is not permissible for the husband to use, spend, or derive any benefit from her mahr without her consent.
However, if the wife willingly and voluntarily, without any pressure, or compulsion, permits her husband to use her mahr, or gifts all or part of it to him, then it is permissible for the husband to benefit from it.
۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔
دلائل:
الھندیة: (304/1، ط: دار الکتب العلمیة)
وَالْمَهْرُ يَتَأَكَّدُ بِأَحَدِ مَعَانٍ ثَلَاثَةٍ: الدُّخُولُ، وَالْخَلْوَةُ الصَّحِيحَةُ، وَمَوْتُ أَحَدِ الزَّوْجَيْنِ سَوَاءٌ كَانَ مُسَمًّى أَوْ مَهْرَ الْمِثْلِ حَتَّى لَا يَسْقُطَ مِنْهُ شَيْءٌ بَعْدَ ذَلِكَ إلَّا بِالْإِبْرَاءِ مِنْ صَاحِبِ الْحَقِّ، كَذَا فِي الْبَدَائِعِ.
فتح القدير: (331/3، ط: دار الفکر)
(وَإِنْ حَطَّتْ عَنْهُ مِنْ مَهْرِهَا صَحَّ الْحَطُّ)؛ لِأَنَّ الْمَهْرَ بَقَاءُ حَقِّهَا وَالْحَطُّ يُلَاقِيهِ حَالَةَ الْبَقَاءِ.
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.