I don't believe you have the full context of the treatment of slaves under the Law:
And thou shalt remember that thou wast a bondman in the land of
Egypt, and the LORD thy God redeemed thee: therefore I command thee
this thing to day."
First of all, the distinction must be made between Hebrew slaves, and slaves of foreigners, or conquered peoples. Hebrew slaves were to be set free at the 7th year,
And if thy brother, an Hebrew man, or an Hebrew woman, be sold unto
thee, and serve thee six years; then in the seventh year thou shalt
let him go free from thee. 13 And when thou sendest him out free from
thee, thou shalt not let him go away empty: 14 Thou shalt furnish him
liberally out of thy flock, and out of thy floor, and out of thy
winepress: of that wherewith the LORD thy God hath blessed thee thou
shalt give unto him
This is compared to Lev. 25:44-46:
Both thy bondmen and thy bondmaids, whom thou shalt have, shall be of
the heathen who are round about you. From them shall ye buy bondmen
45 Moreover of the children of the strangers who sojourn among you,
from them shall ye buy and from their families who are with you, whom
they begot in your land; and they shall be your possession.46 And ye shall take them as an inheritance for your children after
you, to inherit them for a possession. They shall be your bondmen for
ever. But over your brethren, the children of Israel, ye shall not
rule one over another with rigor
A foreign or heathen slave "could be ruled over with rigor"; furthermore, they would remain in the possession of the family. But with a Hebrew slave(Lev. 25:53):
53 And as a yearly hired servant shall he be with him, and the other
shall not rule with rigor over him in thy sight.
Understand also that the slaves, both Hebrew and foreigners, would keep the Sabbath(Deut. 5:15/Ex.23:12) along with the required feasts.
The bottom line is found in Lev. 25:17,
"17 Ye shall not therefore oppress one another; but thou shalt fear
thy God: for I am the LORD your God
In biblical times a person could sell himself, wife, or children to pay a debt. Foreigners were either captured, or those who escaped death through servitude. The Children of Israel were reminded yearly at Passover that they once served "with great rigor", therefore, they were reminded not to do so themselves, especially one to another. One must also remember that abuse, such as putting out an eye or any body part meant the slave went free in exchange(Ex. 21:30)
What the Law did not require is if a man took some means to 'correct' a slave, and if the slave suffered physically, but recovered, a man was not expected to compensate his slave for the 'correction', the slave was "his money". But, if the slave fled his owner, those whom he ran to(Deut. 23:15-16),
" Thou shalt not deliver unto his master the servant which is
escaped from his master unto thee: 16 He shall dwell with thee, even
among you, in that place which he shall choose in one of thy gates,
where it liketh him best: thou shalt not oppress him.
Therefore, the admonition was to "be a good Master", to prevent the slave from running away, and those receiving him/her were admonished not to deliver him/her back.
"He hath showed thee, O man, what is good; and what doth the Lord
require of thee, but to do justly, and to love mercy, and to walk
humbly before the Lord.
In the eyes of God; to say that God 'sanctions' cruelty towards a slave is contrary to the letter and intent of Scripture. The passage quoted has a narrow application, and one that should have seldom been used.