Amr bin Haram (may Allah have mercy on him) said that Jabir bin Zayd (may Allah have mercy on him) was asked: "May a free man marry a slave woman while he is able to pay the dowry of a free woman?" He said: "Only the one who cannot pay the dowry of a free woman may marry a slave woman, provided he fears zina."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13998