Surely oppression maketh a wise man mad; and a gift destroyeth the heart.
Oppression maketh a wise man mad - recurring to the idea, Ecclesiastes 3:16; Ecclesiastes 5:8. Its connection with Ecclesiastes 7:4-6 is, the sight of "oppression" perpetrated by "fools" might tempt the "wise" to call in question God's dispensations, and imitate the folly (i:e., 'madness') described, Ecclesiastes 7:5-6 (Psalms 73:2-3, etc.; Proverbs 23:17).
A gift destroyeth the heart - i:e., the sight of bribery in "places of judgment" (Ecclesiastes 3:16) might cause the wise to lose their wisdom ("heart") (Job 12:6). Hengstenberg explains it, not so well, oppression befools the oppressor, thought once he was wise, and a bribe destroys the understanding. Hence, the happiness of "fools" (Ecclesiastes 7:6) is so short: they work their own ruin. But Ecclesiastes 7:8, latter clause, and Ecclesiastes 3:16; Ecclesiastes 5:8, confirm the former explanation.