Gemara

(R. Zeira): If a slave married a Bas Yisrael in front of his master, he goes free.

Question (R. Yochanan - Beraisa - R. Meir): If a man wrote a document of Kidushin to his Shifchah, she is Mekudeshes;

Chachamim say, she is not Mekudeshes.

Answer: Rabah bar Rav Shilo taught that a slave is free if his master himself put Tefilin on the slave. Here, he goes free if the master himself married off the slave.

Question: Do Chachamim hold that the master would not cause his slave to sin, but he himself would sin (by marrying his Shifchah without freeing her)?

Answer (Rav Nachman bar Yitzchak): The case is, he told her that the document should free her and be Mekadesh her to him. The Tana'im argue about whether or not 'you are Mekudeshes to me' is a valid expression of freedom.

Gitin 81a (Mishnah - Beis Shamai): If a man divorced his wife and spent the night with her in an inn, he need not give her another Get;

Beis Hillel say, he must give her another Get.

(R. Yochanan): The argument is when witnesses saw that they had Bi'ah;

Beis Shamai hold that a man would have Bi'as Zenus. Beis Hillel hold that he would not (surely, he was Mekadesh her).

Yevamos 22a (Mishnah): A son from any mother exempts his father's wife from Yibum and Chalitzah. The only exceptions are a son from a Shifchah or Nochri.

Kesuvos 72b (Rav): If a man was Mekadesh a girl on Tanai (conditionally) and did Nisu'in with her Stam (and the Tanai was not fulfilled), she needs a Get from him (before she can marry someone else);

(Shmuel): She does not need a Get.

(Abaye): Rav does not hold that since he did not mention the Tanai at the time of Nisu'in, this shows that he pardoned it. Rather, a man avoids Bi'as Zenus.

Rishonim

Rif and Rosh (Yevamos 5a and 2:3): The Mishnah says that a child from a Shifchah or Nochris does not exempt from Yibum and Chalitzah. Rabanan say that just like the Nochris is not his own, also the Shifchah. However, if one had Bi'ah with his own Shifchah, even though she does not have a Get Shichrur, the child is his. A man avoids Bi'as Zenus. (Surely, he freed her before Bi'ah.)

Rosh: R. Natrunai Ga'on learns from Gitin 39b. Even Chachamim agree that a man does not sin and he intended to free her, just he erred about what is a valid expression of freedom. Therefore, if one had Bi'ah with his Shifchah, whether it was like marriage or like Bi'as Zenus, surely he freed her beforehand. The Rambam disagrees. I disagree with the Rambam.

Defense (of Rambam - Magid Mishneh, Hilchos Nachalos 4:6): In Gitin, the man wrote a Shtar Kidushin, showing that he desires Kidushin. In other cases it looks like he desires Zenus. A Beraisa teaches that even when a master has Bi'ah with his Shifchah, the children are slaves.

Yam Shel Shlomo (Yevamos 2:10): If he gave to her a Shtar for freedom and Kidushin, this shows that he did not free her beforehand. Otherwise, we assume that he freed her beforehand. The Magid Mishneh's reasoning is sound, but the Gemara connotes that the governing rule is that a man does not sin if he can fulfill his desire in a permitted way. Perhaps the Rambam disagrees because he holds that one is lashed for Bi'as Zenus, so even if he freed her he has no Chezkas Kashrus. The Rosh holds like the Ra'avad and most Ge'onim, that one may designate a Pilegesh (concubine) for himself. The Beraisa discusses when we know that he did not free her. When we do not know, we assume that he did.

Rambam (Hilchos Nachalos 4:6): If Levi fathered a child from his Shifchah and treated him like a son, or said that he is his son and his mother was freed:

If Levi is a Chacham or is established to observe Mitzvos meticulously, the son inherits him. Nevertheless, he may not marry a Bas Yisrael until he proves that his mother was freed before he was born, for she was Muchzekes to be a Shifchah.

Question: Also a proof that he freed the son permits the son to a Bas Yisrael!

Answer (Rashba 1183): This is true. However, if the son will marry a convert, the son's daughter will be permitted to a Kohen (according to R. Eliezer ben Yakov) only if he was born after his mother was freed. Perhaps the Rambam relies on the father's morality only if he married the Shifchah, but if he merely had Bi'ah with we are concerned lest his Yetzer ha'Ra overpowered him.

Magid Mishneh: The Gemara does not distinguish between his Shifchah and someone else's. The Rambam explains that just like we do not distinguish between one Nochris and another, we do not distinguish between one Shifchah and another. Even if he freed her, in any case the Bi'ah was Zenus for she was single. Therefore, the Chazakah that a man avoids Bi'as Zenus does not apply, unless you will say that he had Bi'ah l'Shem Kidushin, and another who has Bi'ah with her is Chayav for Eshes Ish. This is absurd. If there were no witnesses, how can she be married?! Rather, surely it was Bi'as Zenus. Nothing suggests that she was freed. Further, Chachamim said that a man avoids Bi'as Zenus only when there is or was Kidushin. If a Panuy and Penuyah (a single man and a single woman) had Bi'ah, even in front of witnesses, she does not need a Get because they were not discussing Kidushin and he did not tell witnesses that he is Mekadesh her. The Rambam relies on the father's Kashrus regarding inheritance, because the father has a Migo (he could achieve his goal without lying, so we assume that he is not lying). He could give his property to anyone he wants, even a slave. His Chezkas Kashrus offsets the Chazakah that she was a Shifchah, so the Migo remains. Even though Levi transgressed Bi'as Zenus, this is not as severe as the Isur of a Shifchah. If Levi has no Chezkas Kashrus, the Migo does not override the Chazakah (that she was a Shifchah).

Poskim

Rema (CM 279:6): If Levi fathered a child from his Shifchah and treated him like a son, or said that he is his son and his mother was freed, some say that she must do Chalitzah.

Rema (EH 4:12): Surely, if one married his Shifchah, we are concerned for Kidushin.

Gra (28): This is like the Ramah, who says that the Kidushin is only Safek.

Beis Shmuel (13): Even though we hold that she needs a Get of freedom, perhaps someone else received it on her behalf and she did not know.

Shulchan Aruch (15:10): Some say that if a man had Bi'ah with his Shifchah, the child is his.

Chelkas Mechokek (4): Opinions differ when he had Bi'as Zenus. If he married her, we said above (4:12) that he surely freed her.

Rema: Even if he says that he intended for Zenus, he is not believed.