وَمَنْ لَّمْ يَسْتَطِعْ مِنْكُمْ طَوْلًا اَنْ يَّنْكِحَ الْمُحْصَنٰتِ الْمُؤْمِنٰتِ فَمِنْ مَّا مَلَكَتْ اَيْمَانُكُمْ مِّنْ فَتَيٰتِكُمُ الْمُؤْمِنٰتِۗ وَاللّٰهُ اَعْلَمُ بِاِيْمَانِكُمْ ۗ بَعْضُكُمْ مِّنْۢ بَعْضٍۚ فَانْكِحُوْهُنَّ بِاِذْنِ اَهْلِهِنَّ وَاٰتُوْهُنَّ اُجُوْرَهُنَّ بِالْمَعْرُوْفِ مُحْصَنٰتٍ غَيْرَ مُسٰفِحٰتٍ وَّلَا مُتَّخِذٰتِ اَخْدَانٍ ۚ فَاِذَآ اُحْصِنَّ فَاِنْ اَتَيْنَ بِفَاحِشَةٍ فَعَلَيْهِنَّ نِصْفُ مَا عَلَى الْمُحْصَنٰتِ مِنَ الْعَذَابِۗ ذٰلِكَ لِمَنْ خَشِيَ الْعَنَتَ مِنْكُمْ ۗ وَاَنْ تَصْبِرُوْا خَيْرٌ لَّكُمْ ۗ وَاللّٰهُ غَفُوْرٌ رَّحِيْمٌ ࣖ ( النساء: ٢٥ )
Wa mal lam yastati' minkum tawlan ai yankihal muhsanaatil mu'minaati famimmaa malakat aimaanukum min fatayaatikumul mu'minaat; wallaahu a'lamu bi eemaanikum; ba'dukum mim ba'd; fankihoohunna bi izni ahlihinna wa aatoohunna ujoorahunna bilma'roofi muhsanaatin ghaira musaa fihaatinw wa laa muttakhizaati akhdaan; fa izaaa uhsinna fa in ataina bifaahi shatin fa'alaihinnna nisfu maa 'alal muhsanaati minal 'azaab; zaalika liman khashiyal 'anata minkum; wa an tasbiroo khairul lakum; wallaahu Ghafoorur Raheem (an-Nisāʾ 4:25)
And whoever among you cannot [find] the means to marry free, believing women, then [he may marry] from those whom your right hands possess of believing slave girls. And Allah is most knowing about your faith. You [believers] are of one another. So marry them with the permission of their people and give them their due compensation [i.e., mahr] according to what is acceptable. [They should be] chaste, neither [of] those who commit unlawful intercourse randomly nor those who take [secret] lovers. But once they are sheltered in marriage, if they should commit adultery, then for them is half the punishment for free [unmarried] women. This [allowance] is for him among you who fears affliction [i.e., sin], but to be patient is better for you. And Allah is Forgiving and Merciful. (An-Nisa  : 25)
1 Mufti Taqi Usmani
If one cannot afford to marry the free Muslim women, then (he may marry) the one you people own of your Muslim girls. Allah knows best about your faith. You are similar to each other.So, marry them with the permission of their masters, and give them their dues, as recognized, they being bound in marriage, not going for lust, nor having paramours. So, once they have been bound in marriage, then, if they commit a shameful act, they shall be liable to half of the punishment prescribed for the free women. That is for those of you who apprehend to indulge in sin. But that you be patient is better for you. Allah is Most-Forgiving, Very-Merciful.