Allowing Khul` and the Return of the Mahr in that Case
When the spouses have irreconcilable differences wherein the wife ignores the rights of the husband, dislikes him and becomes unable to live with him any longer, she is allowed to free herself (from married life) by giving him back what he had given her (in gifts and Mahr). There is no sin on her in this case nor on him if he accepts such offer. This is why Allah said:
[وَلاَ يَحِلُّ لَكُمْ أَن تَأْخُذُواْ مِمَّآ ءَاتَيْتُمُوهُنَّ شَيْئًا إِلاَّ أَن يَخَافَآ أَلاَّ يُقِيمَا حُدُودَ اللَّهِ فَإِنْ خِفْتُمْ أَلاَّ يُقِيمَا حُدُودَ اللَّهِ فَلاَ جُنَاحَ عَلَيْهِمَا فِيمَا افْتَدَتْ بِهِ]
(And it is not lawful for you (men) to take back (from your wives) any of what you gave them, except when both parties fear that they would be unable to keep the limits ordained by Allah (e.g., to deal with each other on a fair basis). Then if you fear that they would not be able to keep the limits ordained by Allah, then there is no sin on either of them if she gives back.)
Sometimes, the woman has no valid reason and she still asks for her marriage to be ended. In this case, Ibn Jarir reported that Thawban said that Allah's Messenger said:
«أَيُّمَا امْرَأَةٍ سَأَلَتْ زَوْجَهَا طَلَاقًا فِي غَيْرِ مَا بَأْسٍ، فَحَرَامٌ عَلَيْهَا رَائِحَةُ الْجَنَّـة»
(Any woman who asks her husband for divorce without justification, then the scent of Paradise will be forbidden for her.)
At-Tirmidhi recorded this Hadith and stated that it is Hasan.
Ibn Jarir said that the Ayah (2:229) was revealed about Thabit bin Qays bin Shammas and his wife Habibah bint `Abdullah bin Ubayy bin Salul. In his Muwatta', Imam Malik reported that Habibah bint Sahl Al-Ansariyah was married to Thabit bin Qays bin Shammas and that Allah's Messenger once went to the Fajr (Dawn) prayer and found Habibah bint Sahl by his door in the dark. Allah's Messenger said, "Who is this'' She said, "I am Habibah bint Sahl, O Messenger of Allah!'' He said, "What is the matter'' She said, "I and Thabit bin Qays'', meaning, (she can no longer be with) her husband. When her husband Thabit bin Qays came, Allah's Messenger said to him:
«هذِهِ حَبِيبَةُ بِنْتُ سَهْلٍ قَدْ ذَكَرَتْ مَا شَاءَ اللهُ أَنْ تَذْكُر»
(This is Habibah bint Sahl, she said what Allah has permitted her to say.)
Habibah also said, "O Messenger of Allah! I still have everything he gave me.'' Allah's Messenger said:
(Take it from her.) So, he took it from her and she remained in her family's house.''
This was reported by Ahmad, Abu Dawud and An-Nasai.
Al-Bukhari reported that Ibn `Abbas said that the wife of Thabit bin Qays bin Shammas came to the Prophet and said, "O Messenger of Allah! I do not criticize his religion or mannerism. But I hate committing Kufr in Islam (by ignoring his rights on her).'' Allah's Messenger said:
«أَتَرُدِّينَ عَلَيهِ حَدِيقَتَه»
(Will you give him back his garden)
She said, "Yes.'' Allah's Messenger said:
«اقْبَلِ الْحَدِيقَةَ وَطَلِّقْهَا تَطْلِيقَة»
(Take back the garden and divorce her once. )
An-Nasa'i also recorded it.