(… وَالْمُحْصَنَاتُ مِنَ النِّسَاءِ ) (And married women …): You are prohibited from marrying women who are already married,
(… إِلَّا مَا مَلَكَتْ أَيْمَانُكُمْ …) (…except those your right hands possess…): They are the women who are acquired through legal war. Slavery was not only prevalent in the Arabian Peninsula, but in all societies. The whole world was suffering from this issue of slavery, so Islam came to finalize slavery by freeing slaves.
Anyway, (The right hand possesses) is a result of the battles or laws which were prevailed at that time, so that anyone today cannot consider a servant woman in his house as a right hand possess. We are not allowed to legislate according to our own opinions. The legislation is what Allah (swt) and his messenger Muhammad (saws) stated. Therefore, we cannot apply a ruling related to a specific time or to something that no longer exists, because in this case the ruling is invalid.
Accordingly, one may ask: Why then this ruling does exist in the Holy Quran, whereas the Holy Quran is assigned to all times and places? Indeed, we do not know, may be the slavery will appear again in the world, has it ever occurred to anyone in the world that there would be crimes, terrorism, extremism, slaughter and killing, as that we see today, under Islamic or religious slogans? So the religious laws and rulings do not come according to anyone’s desires, but rather as Allah (swt) wills, and He is the All-Wise, All-Knowing, All-Aware, Who created man and knows what is right for him at all times and the Holy Qur’an is:
كِتَابٌ أُحْكِمَتْ آيَاتُهُ
(a Book whose verses are perfected) (Hud: 1)
The words of Allah (swt) , The All-Wise:
لَّا يَأْتِيهِ الْبَاطِلُ مِن بَيْنِ يَدَيْهِ وَلَا مِنْ خَلْفِهِ ۖ تَنزِيلٌ مِّنْ حَكِيمٍ حَمِيدٍ
(Falsehood cannot approach it from before it or from behind it; [it is] a revelation from Wise and Praiseworthy.) (Fussilat: 42)
Whatever attempts are made to distort the principles, rulings and laws of Islam, they will be doomed to failure because Allah (swt) has ensured its preservation:
إِنَّا نَحْنُ نَزَّلْنَا الذِّكْرَ وَإِنَّا لَهُ لَحَافِظُونَ
(Indeed, it is We who sent down the Qur’an and indeed, We will be its guardian.) (Al-Hijr: 9)
(… أُحِلَّ لَكُم مَّا وَرَاءَ ذَٰلِكُم…) (…And lawful to you are beyond these …): You are allowed to marry women other than the prohibited types mentioned before.
(… أَن تَبْتَغُوا بِأَمْوَالِكُم …) (…[provided] that you seek them [in marriage] with [gifts from] your property, desiring chastity,…): The dowry must be paid, as it is a gift given to the wife and represents a kind of value and moral related to marriage. However, there is nothing wrong with you if the wife gives up part of the dowry of what you agreed upon.