Rashi 39a, DH Avdo, writes that this is learned from a Gezeirah Shavah, from the Din of the Torah that a wife is divorced through a Get. Devarim 24:3 states (concerning divorcing a wife), "And he shall write for her (Lah) a Sefer Kerisus." Vayikra 19:20 states (concerning a maidservant), "Or her freedom was not given for her (Lah)."

Because the Torah writes "Lah" in both cases, we learn from this that freedom may be given to the maidservant in the same manner that a woman is divorced, namely by giving her a written document. An Eved, like a maidservant, is freed in the same way, by writing to him a Shtar Shichrur.

Kol Tuv,

Dovid Bloom

1) I think it should just be added to the above comments that while the Gezeirah Shavah of "Lah, Lah" cited by the Gemara (Gitin 39b) teaches us that it is possible to free an Eved Kena'ani with a Shtar, this does not mean that he is compared entirely to a woman and her divorce. This is why the Mishnah in Kidushin 22b states that the Eved Kena'ani can also be freed with an exchange of money. This clearly is not similar to a wife, who cannot be divorced by payment.

2) This is also why Shmuel says (Gitin 38a) that if someone is Mafkir his Eved, the Eved goes free and does not require a Shtar. Obviously, the same thing does not apply to a wife. See Tosfos to Yevamos 48a, DH ha'Mafkir, who writes that Hefker means that the owner is relinquishing his financial rights to the Eved, so this works orally according to Shmuel, without a Shtar.

3) A Talmid Chacham referred me to Tosfos in Kidushin, beginning of 3b, who writes that we learn from a Hekesh to include new ways of making a Kinyan, but not to exclude ways. Similarly, we may say that the Gezeirah Shavah of "Lah, Lah" teaches that it is possible to free an Eved with a Shtar but it does not mean that it must be done only that way.

Kol Tuv,

Dovid Bloom