Masruq (may Allah have mercy on him) said: When you marry a free woman while having a slave wife, that is the divorce of the slave woman. It is like carrion, which is eaten in a time of necessity; when he no longer needs it, he should refrain.
We hold only what we have narrated on this from Sayyiduna Ali and Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both), and success is from Allah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14009