Albert Barnes Commentary Ecclesiastes 5:6

"Suffer not thy mouth to cause thy flesh to sin; neither say thou before the angel, that is was an error: wherefore should God be angry at thy voice, and destroy the work of thy hands?" — Ecclesiastes 5:6 (ASV)

Suffer not thy mouth ... - that is, do not make rash vows which may later be the cause of evasion and prevarication, and remain unfulfilled.

Before the angel - The Septuagint and some other versions translate "before the face of God," meaning a spiritual being representing the presence of God, a minister of divine justice (Exodus 23:21), one such as inflicted judgment upon David (2 Samuel 24:17). Others, with less probability, understand the angel to be a priest, and refer to Malachi 2:7.