Yoreh Deah Siman 267 Seif 16

 

טז מֶלֶךְ שֶׁגָּזַר שֶׁכָּל מִי שֶׁלֹּא יִתֵּן הַמַּס הַקָּצוּב עַל כָּל אִישׁ וְאִישׁ יִשְׁתַּעְבֵּד לָזֶה שֶׁנָּתַן הַמַּס עַל יָדוֹ, הֲרֵי זוֹ מֻתָּר לְהִשְׁתַּמֵּשׁ בּוֹ יוֹתֵר מִדַּאי, אֲבָל לֹא כְּעֶבֶד. וְאִם אֵינוֹ נוֹהֵג כַּשּׁוּרָה, מֻתָּר לְהִשְׁתַּמֵּשׁ בּוֹ כְּעֶבֶד.

Seif 16

 

If a king decrees that all who do not pay taxes be enslaved to the person who pays the head tax for him, a person who pays the head tax for someone may use that person for labor beyond the ordinary measure. He may not, however, use him as a slave. If, however, that person does not conduct himself properly, he may utilize him as a slave.