Midrasch zu Schemot 21:4

3) (Vayikra 25:44) ("And your man-servant and your maid-servant, which will be to you, from the nations, which are around you; of them shall you buy a man-servant and a maid-servant.") Lest you say: If You forbade all of these to us, who shall serve us? It is, therefore, written "And your man-servant and your maid-servant, which will be to you from the nations." What is the intent of this? Whence is it derived that if a Jew lived with his maid-servant and begot a son by her you are permitted to make him your servant? From "which will be to you." "from the nations which are around you": and not from the Canaanites in the land, (whom it is a mitzvah to destroy). "from them shall you buy a man-servant and a maid-servant": "a man-servant and a maid-servant": man-servant is being compared to maid-servant. Just as there is no kiddushin (betrothal) for a maid-servant (with a Jew) (viz. Shemoth 21:4), so, there is no kiddushin for a man-servant (with a Jewess).