הלכה על קידושין 44:6

From the laws of the commandment is that which they, may their memory be blessed, said (Kiddushin 22b) that he may acquire [his freedom] though money, a contract, or [an injury to] the main limbs that will not grow back - and they are twenty-four. And even though the law pertaining to him is to go out for the main limbs that do not grow back, he [still] requires a contract of manumission from the [master]; and we force the master to write [it] for him, since he caused him to lack one of the twenty-four well-known limbs. And they, may their memory be blessed, said (Gittin 43b) that [in a case of] one who sells his slave to gentiles or Kuthites, or even to a resident alien, [the] slave goes out to freedom. And so [too, in the case of] an Israelite who lives in the Land [of Israel] who sold him to an Israelite who lives outside of the Land, in order that he take him outside of the Land - [the] slave goes out to freedom. And similarly, they, may their memory be blessed, said (Gittin 40a) that if his master marries him to a free woman, or if lays tefillin on him, or [if] his master tells him to read three verses in a Torah scroll before the congregation or anything that is similar to these things about which only free men are obligated, [the slave] goes out to freedom with these and we force his master afterwards to write him a contract of manumission. And therefore the master must be careful not to do any of these things at all, so that he will not violate this positive commandment, unless he did this for the honor of [another] commandment, as we explained. And the laws of the writing of the contract, its instruction and the rest of its details are in Kiddushin and Gittin (see Tur, Yoreh Deah 267 and Tur, Choshen Mishpat 197).