The Quran

Preface

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

Yusuf Ali

Let those who find not the wherewithal for marriage keep themselves chaste, until God gives them means¹ out of His grace. And if any of your slaves ask for a deed in writing (to enable them to earn their freedom for a certain sum), give them such a deed² if ye know any good in them: yea, give them something yourselves out of the means which God has given to you. But force not your maids³ to prostitution when⁴ they desire chastity, in order that ye may make a gain in the goods of this life. But if anyone compels them, yet, after such compulsion, is God, Oft-Forgiving, Most Merciful (to them).⁵

A Muslim marriage requires some sort of a dower for the wife. If the man cannot afford that, he must wait and keep himself chaste. It is no excuse for him to say that he must satisfy his natural cravings within or outside marriage. It must be within marriage.
The law of slavery in the legal sense of the term is now obsolete. While it had any meaning, Islam made the slave’s lot as easy as possible. A slave, male or female, could ask for conditional manumission by a written deed fixing the amount required for manumission and allowing the slave meanwhile to earn money by lawful means and perhaps marry and bring up a family. Such a deed was not to be refused if the request was genuine and the slave had character. Not only that, but the master is directed to help with money out of his own resources in order to enable the slave to earn his or her own liberty.
Where slavery was legal, what is now called the “white slave traffic” was carried on by wicked people like ʿAbdullah ibn Ubai, the Hypocrite leader at Medīna. This is absolutely condemned. While modern nations have abolished ordinary slavery, the “White Slave Traffic” is still a big social problem in individual States. Here it is absolutely condemned. No more despicable trade can be imagined. (R).
I have translated “in” (literally, “if) by “when” because this is not a conditional clause but an explanatory clause, explaining the meaning of “force”. “Forcing” a person necessarily means that it is against the wish or inclination of the person forced. Even if they were to give a formal consent, it is not valid because the persons concerned are in (legal, or now) economic slavery.
The poor unfortunate girls, who are victims of such a nefarious trade, will yet find mercy from God, whose bounties extend to the lowest of His creatures.