Yoreh Deah Siman 267 Seif 14

יד אֵין עֶבֶד עִבְרִי נוֹהֵג אֶלָּא בִּזְמַן שֶׁהַיּוֹבֵל נוֹהֵג. וְהָנֵי מִלֵּי לְמִקְנְיֵהּ גּוּפֵיהּ לִשְׁאָר דִּינֵי עֶבֶד עִבְרִי, אֲבָל הֵיכָא דְּקַנְיֵהּ שַׁבַּאי לְמַעֲשֶׂה יָדָיו בְּדִינָא דְּמַלְכוּתָא, כְּגוֹן שֶׁנִּשְׁבָּה בְּמִלְחָמָה שֶׁמְּכָרוּהוּ גּוֹבֵי הַמַּס, הָדָר מָצֵי לְזַבּוּנֵי יִשְׂרָאֵל לְמַעֲשֵׂה יָדָיו, וּמָצֵי הַאי עֶבֶד לְמֵיהַב לְרַבֵּיהּ יִשְׂרָאֵל דְּמֵי מִקְנָתוֹ אוֹ דְּמֵי שָׁוְיוֹ, וְנָפִיק, וְיַד הָאָדוֹן עַל הַתַּחְתּוֹנָה לִקַּח הַפָּחוֹת.

 

There is no authority for Hebrew Slaves except in the era that the Jubilee was observed. This is said in the terms of purchase (like from a Beth Din) relating to all the rules governing Hebrew Slaves. However, in the situation when his (Hebrew Slaves) talent is sold under the secular law, such as he was detained in war or sold by tax authorities, then he may employ (The Hebrew Slave) for that talent only.  The  (Hebrew Slave) serves his Jewish Master for the amount paid or the value of acquiring him, and he (Hebrew Slave) is released. The hand of the Master is at a loss to accept the least.