Al-Khul‘ is the separation of a husband from his wife, while receiving a return (of monetary gain from her) and using specific statements (to bring about the dissolution of the marriage).
Al-Khul‘ (literally removal or shedding off) was termed with this (descriptive named) because the woman in essence seeks to remove herself from her husband as she would remove or shed off a garment since spouses are the garments of one another.
Allaah, the Most High says (what means): “It is made lawful for you to have sexual relations with your wives on the night of fasting. They are a Lîbaas (i.e. body cover) for you and you are a Libaas for them.” [Surah Al-Baqarah: 187]
It is commonly held that marriage is what binds the spouses together and cultivates a relationship built upon Ma‘roof (righteousness in all of its forms). Stemming from this relationship a family takes form and a new generation is produced.
Allaah the Most High says: “And among His Signs is this, that He created for you wives from among yourselves, that you may find repose in them, and He has put between you affection and mercy. Verily, in that are indeed signs for a people who reflect.” [Surah Ar-Room: 21]
When this (relationship) is not attained in the marriage – affection and repose is not achieved for one or both of the spouses’ and the situation is foreboding with little chance of reconcilement then the husband is ordered to release her in kindness.
Allaah the Most High says: “…either you retain her on reasonable terms or release her with kindness.” [Surah Al-Baqarah: 229]
“But if they separate (by divorce), Allaah will provide abundance for every one of them from His Bounty. And Allaah is Ever All Sufficient for His creatures' need, All Wise.” [Surah An-Nisaa: 130]
But if the husband finds repose and comfort, but the wife does not find the same in him as a result of her disliking his traits, physical appearance, deficiency of religion or she fears that she would displease Allaah by not fulfilling his (husband’s) rights and needs; then in this case she may request a separation and dissolution from him while extending to him an offer of monetary gain so as to liberate herself from him.
Allaah the Most High says: “…except when both parties fear that they would be unable to keep the limits ordained by Allaah (i.e. 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 Allaah, then there is no sin on either of them if she gives back (the Mahr or a part of it) for her (to achieve Al-Khul').” [Surah Al-Baqarah: 229]
That if the husband or the wife know that they will not fulfill the ordainments of Allaah towards each other if they remain in union, leading to the transgression of the husband on the woman, causing her to disobey him (refuse his cohabitation), then there should be no harm in the woman liberating herself from him by appeasing him financially (to gain discharge). There is also no harm in the husband accepting the remuneration and then letting her go.
The Ruling of al-Khul‘:
The woman can free herself from her husband completely in a just process that is beneficial to both parities. The husband is to cooperate with her in this regard. If he loves her then it is preferred for her to remain with him and have Sabr and not seek to separate.
Khul‘ is Mubaah (permitted) when the proper reasons – alluded to in the previous verses are found. These reasons include both parties fearing that if they remain in union that they will not fulfill the ordainments of Allaah towards each other. (On the other hand) it is Makrooh (disliked) and in the opinion of some scholars it is Haraam (prohibited) to seek Khul‘ without due cause. To this effect, Allaah’s Messenger (saws) said:
“Any woman who asks her husband for a divorce (khul‘) without due cause, then the scent of Paradise is Haraam for her (i.e. she will not smell it).”
[Reported by the five except an-Nasaa’ee] 
Shaikh Taqee-ud-Deen (Ibn Taimiyyah) said: “The Khul‘ which is in accordance to the Sunnah is that the woman has an abhorrence to the man (her spouse). She ransoms herself from him in the same way a captive is ransomed.” 
If the husband hates her but retains her hoping that she will seek to ransom herself (in so doing producing a profit for himself) then he is Dhaalim (Oppressive and tyrannical). Any monetary gain he receives is thus Haraam and the Khul‘ is invalid (i.e. he is to divorce her and not do Khul‘). Allaah, the Most High says:
“O you who believe! You are forbidden to inherit women against their will, and you should not treat them with harshness, that you may take away part of the Mahr you have given them…” [Surah An-Nisaa: 19]
This means that no harm should be inflicted while living together so as to compel her to give back the dowry or not ask for a marital right that she is due by her husband unless it is a censure from the husband to a woman who has fornicated and he seeks to take back what he had graced her with. This instance is sanctioned by Allaah. Allaah says:
“O you who believe! You are forbidden to inherit women against their will, and you should not treat them with harshness, that you may take away part of the Mahr you have given them, unless they commit open illegal sexual intercourse.” [Surah An-Nisaa: 19]
Ibn ‘Abbaas (raa) said explaining this verse:
“This verse pertains to a man who has a woman whom he hates to be in companionship with but he owes her payment of her dowry. So he harms her so that she will relinquish seeking the dowry from him so as to free herself from him. Allaah has prohibited this action when He said: ‘…unless they commit open illegal sexual intercourse.’ Zinaa (adultery) is what is implied. In this instance the husband can take back what he had given her as dowry. He is to compel her to relinquish all the dowry he gave to her and then grant her the Khul‘.”
The Evidence for the Permissibility of al-Khul‘:
The evidence for the permissibility of al-Khul’, when there is a valid cause for it, can be found in the Qur’aan and Sunnah and the Ijmaa‘ (consensus of the scholars):
As for the Qur’aan, we find evidence in the previously mentioned verse wherein Allaah says:
“…except when both parties fear that they would be unable to keep the limits ordained by Allaah (i.e. 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 Allaah, then there is no sin on either of them if she gives back (the Mahr or a part of it) for her (to achieve Khul').” [Surah Al-Baqarah: 229]
As for the Sunnah, then we find in the authentic narrations:
Narrated Ibn 'Abbas (raa): The wife of Thaabit bin Qais came to the Prophet and said, "O Allaah's Apostle! I do not blame Thaabit for defects in his character or his religion, but I, being a Muslim, dislike to behave in un-Islamic manner (if I remain with him)." On that Allaah's Apostle said (to her): "Will you give back the garden which your husband has given you (as Mahr)?" She said, "Yes." Then the Prophet said to Thaabit, "O Thaabit! Accept your garden, and divorce her once." [Al-Bukharee] 
As for Ijmaa’, then we note that Ibn ‘Abdul-Barr (rahimahullaah) stated: “We do not know of anyone who disagreed (on the validity of Khul‘) except al-Muznee - he is the only one who has stated that the verse [Al-Baqarah: 229] was abrogated by: ‘But if you intend to replace a wife by another and you have given one of them a Qintar (of gold i.e. a great amount) as Mahr, take not the least bit of it back; would you take it wrongfully without a right and (with) a manifest sin?’ [Surah An-Nisaa: 20]”
The following Shuroot (prerequisites) must exist to ensure the validity of al-Khul‘:
1. A valid (legally acceptable) remuneration must be extended
2. The remuneration must be to the spouse who can (is the one to) agree to the separation
3. The husband is not allowed to treat the woman harshly – except where deemed proper – so as to force her to pay him
4. The separation is to be stated with the expressed articulation of Khul‘. To use the statement of Talaaq or that which is known to be a statement of Talaaq with the intention of Talaaq is to be counted as Talaaq (and not a khul‘), which eliminates the man’s power to reclaim her as his wife. He may remarry her with a new ‘Aqd (marital contract and Dowry) even if she has not taken a husband other than him as long as he has not pronounced Talaaq thrice upon her
5. If the man pronounces the separation with the statement as that of Khul‘ or Faskh (dissolution) or ransom (yourself from me) – and does not intend divorce then it is a dissolution of marriage that is not incorporated in the Talaaq count. This is reported from Ibn ‘Abbaas. His proof was from the following verses.
Allaah the Most High says: “The divorce is twice…” [Surah Al-Baqarah: 229]
Then later on Allaah said: “And if he has divorced her (the third time), then she is not lawful unto him thereafter until she has married another husband.” [Surah Al-Baqarah: 230]
So notice Allaah mentioned two divorce pronouncements, then mentioned the Khul‘ then mentioned another divorce pronouncement. Therefore we assess that Khul‘ does not count as a Talaaq pronouncement or else the count would be four (exceeding the limit of three).