John Calvin 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 go out from you. Moses does not reply to this demand, because he knew that God's design was otherwise; and God had justly left him in ignorance about what He did not yet wish him to know. Therefore, there is no reason why Moses should be accused of bad faith when he faithfully fulfilled the charge committed to him, although he was silent about what he was not ordered to declare, even about that which God wished to be concealed from the tyrant.

But the holy Prophet, aroused to pious indignation by the king’s treachery, does not immediately remove the plague but waits until the next day. Moreover, he severely denounces that, if the king should persist in deceit, its punishment awaited him. This great magnanimity he had derived from the miracles, for, having experienced God's unconquerable power in them, he had no cause for fear. For it was an act of extraordinary boldness to openly reproach the tyrant to his face for his falsehoods, and at the same time to threaten him with punishment unless he desisted from them.

But we said before that Moses had not acted from his own human reasoning when he promised Pharaoh what he asked, but that he had spoken so confidently from a special impulse. For the general promise in which God affirms that He will grant the prayers of His servants must not be applied to particular cases, so that they should expect to obtain this or that in a specified manner, unless they have some unique testimony from the word or the Spirit of God.