Charles Ellicott Commentary Exodus 8:29

"And Moses said, Behold, I go out from thee, and I will entreat Jehovah that the swarms of flies may depart from Pharaoh, from his servants, and from his people, to-morrow: only let not Pharaoh deal deceitfully any more in not letting the people go to sacrifice to Jehovah." — Exodus 8:29 (ASV)

And Moses said, Behold ... I will intreat the Lord. —Moses accepted Pharaoh’s second promise, and took no special exception to its condition —“only you shall not go very far away.” He had distinctly stated his own demand, which was for “a three days’ journey into the wilderness” (Exodus 5:3; Exodus 8:27). It was for Pharaoh to settle with himself whether he considered that distance “very far” or not. As he made no clear objection to the distance, Moses was bound to suppose that he allowed it.

Let not Pharaoh deal deceitfully any more. God’s servants must rebuke even kings when they openly break the moral law (1 Samuel 13:13; 1 Samuel 15:16–23; 2 Samuel 12:7–12; 1 Kings 21:20–22; Matthew 14:4; and others). Pharaoh had promised unconditionally to let the people go if the frogs were removed (Exodus 8:8), and had then flagrantly broken his word. Moses was right to rebuke his “deceit.”