Forbidding Women Already Married, Except for Female Slaves

Allah said,

The Ayah means, you are prohibited from marrying women who are already married,

except those whom you acquire through war, for you are allowed such women after making sure they are not pregnant.

Imam Ahmad recorded that Abu Sa'id Al-Khudri said, “We captured some women from the area of Awtas who were already married, and we disliked having sexual relations with them because they already had husbands. So, we asked the Prophet about this matter, and this Ayah was revealed,

Consequently, we had sexual relations with these women.”
This is the wording collected by At-Tirmidhi, An-Nasa'i, Ibn Jarir and Muslim in his Sahih.

Allah’s statement,

{Thus has Allah ordained for you} means, this prohibition was ordained for you by Allah. Therefore, adhere to Allah’s Book, do not transgress His set limits, and adhere to His legislation and decrees.

The Permission to Marry All Other Women

Allah said,

{All others are lawful} meaning, you are allowed to marry women other than the prohibited types mentioned here, as ‘Ata’ and others have stated. Allah’s statement,

meaning, you are allowed to use your money to marry up to four wives and for (the purchase of) as many female slaves as you like, all through legal means,

Allah’s statement,

means, to enjoy them sexually, surrender to them their rightful dowry as compensation. In other Ayat, Allah said,

{And give to the women (whom you marry) their dowry with a good heart}, and,

Prohibiting the Mut'ah of Marriage

Mujahid stated that,

was revealed about the Mut'ah marriage. A Mut'ah marriage is a marriage that ends upon a predetermined date.

In the Two Sahihs, it is recorded that the Leader of the Faithful ‘Ali bin Abi Talib said, “The Messenger of Allah prohibited Mut'ah marriage and eating the meat of domesticated donkeys on the day of Khaybar (battle).”
In addition, in his Sahih, Muslim recorded that Ar-Rabi' bin Sabrah bin Ma'bad Al-Juhani said that his father said that he accompanied the Messenger of Allah during the conquest of Makkah, and that the Prophet said,

«O people! I allowed you the Mut'ah marriage with women before. Now, Allah has prohibited it until the Day of Resurrection. Therefore, anyone who has any women in Mut'ah, let him let them go, and do not take anything from what you have given them.»

Allah’s statement,

{but if you agree mutually (to give more) after the requirement (has been determined), there is no sin on you.} is similar to His other statement,

The meaning of these Ayat is: If you have stipulated a dowry for her, and she later forfeits it, either totally or partially, then this bears no harm on you or her in this case. Ibn Jarir said, “Al-Hadrami said that some men would designate a certain dowry, but then fall into financial difficulties. Therefore, Allah said that there is no harm on you, O people, concerning your mutual agreement after the requirement (has been determined).” meaning, if she gives up part of the dowry, then you men are allowed to accept that. Allah’s statement,

{Surely, Allah is Ever All-Knowing, All-Wise} is suitable here, after Allah mentioned these prohibitions.