The Quran

وَليَستَعفِفِ الَّذينَ لا يَجِدونَ نِكاحًا حَتّىٰ يُغنِيَهُمُ اللَّهُ مِن فَضلِهِ ۗ وَالَّذينَ يَبتَغونَ الكِتابَ مِمّا مَلَكَت أَيمانُكُم فَكاتِبوهُم إِن عَلِمتُم فيهِم خَيرًا ۖ وَآتوهُم مِن مالِ اللَّهِ الَّذي آتاكُم ۚ وَلا تُكرِهوا فَتَياتِكُم عَلَى البِغاءِ إِن أَرَدنَ تَحَصُّنًا لِتَبتَغوا عَرَضَ الحَياةِ الدُّنيا ۚ وَمَن يُكرِههُنَّ فَإِنَّ اللَّهَ مِن بَعدِ إِكراهِهِنَّ غَفورٌ رَحيمٌ

Muhammad Asad

And as for those who are unable to marry,¹ let them live in continence until God grants them sufficiency out of His bounty. And if any of those whom you rightfully possess² desire [to obtain] a deed of freedom, write it out for them if you are aware of any good in them:³ and give them [their share] of the wealth of God which He has given you.⁴ And do not, in order to gain⁵ some of the fleeting pleasures of this worldly life, coerce your [slave] maidens into whoredom if they happen to be desirous of marriage;⁶ and if anyone should coerce them, then, verily, after they have been compelled [to submit in their helplessness], God will be much-forgiving, a dispenser of grace!

I.e., because of poverty, or because they cannot find a suitable mate, or for any other personal reason.
Lit., "whom your right hands possess", i.e., male or female slaves.
The noun kitab is, in this context, an equivalent of kitabah or mukatabah (lit., "mutual agreement in writing"), a juridical term signifying a "deed of freedom" or "of manumission executed on the basis of an agreement between a slave and his or her owner, to the effect that the slave undertakes to purchase his or her freedom for an equitable sum of money payable in instalments before or after the manumission, or, alternatively, by rendering a clearly specified service or services to his or her owner. With this end in view, the slave is legally entitled to engage in any legitimate, gainful work or to obtain the necessary sum of money by any other lawful means (e.g., through a loan or a gift from a third person). In view of the imperative form of the verb katibuhum ("write it out for them"), the deed of manumission cannot be refused by the owner, the only pre-condition being an evidence to be established, if necessary, by an unbiassed arbiter or arbiters - of the slave's good character and ability to fulfil his or her contractual obligations. The stipulation that such a deed of manumission may not be refused, and the establishment of precise juridical directives to this end, clearly indicates that Islamic Law has from its very beginning aimed at an abolition of slavery as a social institution, and that its prohibition in modern times constitutes no more than a final implementation of that aim. (See also next note, as well as note 146 on 2: 177.)
According to all the authorities, this relates (a) to a moral obligation on the part of the owner to promote the slave's efforts to obtain the necessary revenues by helping him or her to achieve an independent economic status and/or by remitting part of the agreed-upon compensation, and (b) to the obligation of the state treasury (bayt al-mal) to finance the freeing of slaves in accordance with the Qur'anic principle - enunciated in 9:60 - that the revenues obtained through the obligatory tax called zakah are to be utilized, among other purposes, "for the freeing of human beings from bondage" (fi 'r-riqab, an expression explained in surah 2, note 146). Hence, Zamakhshari holds that the above clause is addressed not merely to persons owning slaves but to the community as whole. - The expression "the wealth of God" contains an allusion to the principle that "God has bought of the believers their lives and their possessions, promising them paradise in return" (9:111) - implying that all of man's possessions are vested in God, and that man is entitled to no more than their usufruct.
Lit., "so that you might seek out" or "endeavour to attain to".
Lit., "if they desire protection against unchastity (tahassun)", i.e , through marriage (cf. the expression muhsanat as used in 4:24). Most of the classical commentators are of the opinion that the term fatayat ("maidens") denotes here "slave-girls": an assumption which is fully warranted by the context, Hence, the above verse reiterates the prohibition of concubinage by explicitly describing it as "whoredom" (bigha').