Charles Ellicott Commentary Proverbs 30:10

"Slander not a servant unto his master, Lest he curse thee, and thou be held guilty." — Proverbs 30:10 (ASV)

Accuse not a servant—that is, a slave, thus making his already hard life still more intolerable.

And you be found guilty before God of having wronged him, and so have to bear the punishment.