Gemara

Question (Rav Papa): There are Rabanan who pay the head-tax of people, and make them work for them!

Answer (Rava): The king is master over all his subjects. He says that whoever pays Ploni's head-tax may make Ploni work for him.

Rav Se'oram (Rava's brother) used to grab indecent people and make them pull Rava's rickshaw.

Support (Rava - Beraisa): If one does not act properly, you may force him to work for you - "l'Olam Bahem Ta'avodu uv'Acheichem" (forever you will work with Kena'ani slaves and with your brothers)";

Suggestion: Perhaps this is even if he acts properly!

Rejection: "Uv'Acheichem Bnei Yisrael (you may not work harshly)."

Yevamos 46a - Question (Rav Papa): If one paid Ploni's tax and used him for a slave, does he need a document of freedom to go free?

Answer (Rava citing Rav Sheshes): He was the king's slave, and the king said that one who does not pay his own tax works for the one who paid it.

Gitin 38a: We learn that a Nochri can acquire a Yisrael from "they took a captive from us."

Rishonim

The Rif brings the Gemara in Bava Metzia verbatim.

Nimukei Yosef (16a DH Garsinan, citing the Ritva): Rashi explains that the people bought were Nochrim. This is difficult, for if so one may work them excessively! Also, in Bava Metzia this is brought regarding Ribis! Rather, they were Yisre'elim. We asked whether the king can sell a Yisrael to be an Eved Ivri, who is bodily acquired and needs a Get to go free. Or, perhaps it is like a mere debt, and the buyer cannot work with him even like an Eved Ivri more than the debt, lest he transgress subjugating a Yisrael and Ribis. We answered that the king conquered them in war, and left them in his land on condition that one who does not pay the tax can be sold to be a slave. Eved Ivri applies only when Yovel applies, i.e. to leave after six years or through Simanim, and to permit marrying a Shifchah. Even when there is no Yovel, he is a slave and is freed only through a Get or money.

The Rosh brings Rava's defense of his brother's practice.

Hagahos Ashri brings Rava's justification for paying the head-tax of people and making them work for him.

Rambam (Hilchos Gezeilah 5:16): If a king decreed that whoever pays the fixed tax for Ploni, who did not pay it, may make Ploni work for him, and a Yisrael paid the tax for a poor Yisrael, he may make him work exceedingly, for the king's law is law. However, he may not work with him like a slave.

Rambam (Hilchos Malveh 5:17): If a king decreed that whoever pays the fixed tax for Ploni, who did not pay it, may make Ploni work for him, and one paid a Dinar for Ploni's tax, he may make him work for more than a Dinar. The same applies to all similar cases.

Rambam (Hilchos Avadim 1:7): One may not make an Eved Ivri do tasks special for slaves, e.g. carrying one's clothes after him to the bathhouse or removing his shoes. The Isur applies only to an Eved Ivri, who is dispirited by the sale. If a Yisrael was never sold one may use him like a slave, for he works willingly.

Rambam (8): If one does not act properly, one may force him to work for him. If a king decreed that if Ploni does not pay his fixed tax, Ploni must work for one (Levi) who pays it, Levi may make him work exceedingly, but not like a slave. If Ploni does not act properly, one may make him work like a slave.

Rebuttal (Ra'avad): The Gemara did not say so in this case! Rather, people paid a tax of 10 Dinarim for a Yisrael, and made him do 15 Dinarim of work. The Gemara asked whether this is like a loan, and forbidden due to Ribis. It answered that it is not a loan, rather, a sale. The king is authorized to sell his subjects like slaves until they pay the tax.

Kesef Mishneh: The Ra'avad explains that we asked whether the buyer may receive more work than what he paid. The Rambam explains that the buyer worked the people too harshly. The Gemara supports him, for it brings after this the Heter to work harshly with people who act improperly.

Beis Yosef (CM 369 DH u'Mah she'Omar v'Chen): Some texts of the Gemara do not say that the buyer worked with them 'Tfei' (more), for payment of the tax was like a loan. The Shevuos returned the money through their work. The Tur's text did not say 'Tfei'; the Rambam's text did.

Hagahos ha'Gra (73b): The primary explanation is like the Ra'avad (the question was about Ribis). His text said 'Tfei'.

Lechem Mishneh: The Rambam explains that we asked whether making the Yisrael do work worth more than he paid is theft. He learns from 'Tfei'.

Poskim

Shulchan Aruch (CM 369:11): If a king decreed that whoever pays the fixed tax for Ploni, who did not pay it, may make Ploni work for him, and a Yisrael paid the tax for a poor Yisrael, he may make him work exceedingly, for the king's law is law.

SMA (19): This is only if it is accepted and acknowledged in those lands that he is the king. If not, he is like a mere thief.

Shulchan Aruch (ibid): He may not work with him like a slave.

Beis Yosef (DH u'Mah she'Omar u'Bilvad): The Rambam holds that this Isur applies only to an Eved Ivri. The Rambam teaches that since the king's law allows using him since he paid the tax, it is as if he was sold. His law is like an Eved Ivri, so one may not make him work like a slave.

Shulchan Aruch (YD 267:14): Eved Ivri applies only whe Yovel applies. This refers to acquiring him bodily for other laws of Eved Ivri. If captors acquired a Yisrael for his work, through the king's law, e.g. he was captured in war or sold by tax collectors, he can sell him to a Yisrael for his work.

Beis Yosef (DH v'Chosav ha'Ramah): The Ramah (brought in the Tur) explains that the king acquires a Yisrael like a mere worker, not like a slave who is bodily acquired. He explains the Gemara in Yevamos like Rashi, that the king sold Nochrim, unlike the Ritva (above, in Nimukei Yosef).

Ramah (brought in Tur and Shach 25): If he was captured at a time of peace, and not through the king's law, one who buys him does not acquire him at all. He simply goes free.

Ben Lev (3:11, cited by R. Akiva Eiger): This is like Rashi, who says that Gitin 38a teaches that a Nochri can acquire an Eved Kena'ani. The Rashba explains that it teaches that he can acquire a Yisrael. In any case, the slave should need to pay back the one who redeemed him! Sometimes Levi need not pay one who paid Levi's debt, but when Levi was sure to lose, he must pay! Even if this is an enactment, it applies here.

Shulchan Aruch (ibid): The slave can pay his Yisrael master what he paid for him or his worth, and he goes free. The master has the lower hand, and receives the lesser amount.

Shach (26,27): The slave can pay right after he was sold, or afterwards. If he does not pay, he serves the buyer, even if this is worth more than what the buyer paid. The redemption does not decrease according to how long the slave already served, for Eved Ivri does not apply nowadays.

Shulchan Aruch (15): If people do not act properly, one may use force to make them to work for him.

Shulchan Aruch (16): If a king decreed that whoever pays the fixed tax for Ploni, who did not pay it, may make Ploni work for him, one who does so may make him work exceedingly, but not like a slave. If Ploni does not act properly, one may make him work like a slave.

Shach (29): A Yisrael bought from the king does not need a Get to go free, for the king did not own him bodily. A Nochri sold by the king is an Eved Kena'ani (Sa'if 18).

Gra (29*): According to the Rambam, I do not understand what the question was. The Rambam (Hilchos Gezeilah 5:16) and Shulchan Aruch (CM 369:11) explain this law regarding the validity of the king's laws.