As comments have pointed out, the distinction between sentencing someone to slavery and sentencing them to prison under a convict leasing system is in some ways a subtle one. Nonetheless, the document shown in the question does reflect the specific way in which Southern states originally interpreted the Thirteenth Amendment, adopted in late 1865. This continuation of slavery in a new form was a central factor which led to the Civil Rights Act of 1866 and then ultimately the Fourteenth Amendment, adopted in July 1868.
In an NYU Law Review article, James Gray Pope quotes William Higby of California, who began urging passage of the Fourteenth Amendment in February of 1866 specifically because of the Punishment Clause in the Thirteenth.
"Such a provision operates equally upon all classes, but the judge could discriminate, and could say to the white offender, ‘Go to the State prison,’ while he could say to the black man, ‘Go into slavery.’"
Higby warned that Southern states were currently engaging in
such practices, and that Congress could do nothing about it
“because those States are acting under the amendment of the
Constitution, and can pass such laws in spite of anything which
we may do in this Hall, and you leave slavery sealed upon the
So while I've not yet pinpointed the answer to the question, this is the historical context. Explicitly sentencing black people to slavery was arguably legal until July 1868. De facto, it continued long after that but was no longer to be called "slavery".