John Calvin Commentary Exodus 8:28

"And Pharaoh said, I will let you go, that ye may sacrifice to Jehovah your God in the wilderness; only ye shall not go very far away: entreat for me." — Exodus 8:28 (ASV)

And Pharaoh said, I will let you go. When he sees that his delays and tactics are of no use to him, he professes complete obedience. This was not because he then intended to deceive and lie, being restrained by fear, but only because, overwhelmed with a present sense of his calamity, he dared not defy God.

Therefore (as I said before), he did not so much wish intentionally to appease and thwart Moses with falsehood, as he deceived himself. For we must observe that (like one holding a wolf by the ears) he was forced to promise the dismissal of the people, whom he kept to his own great harm.

And this is why he entrusts himself to their prayers, for necessity compelled him to implore God’s pardon and peace, although he might also have craftily desired to win their affection for himself under the guise of religion.

For by this anxious concern for himself, he reveals his lack of confidence. Finally, by requesting their prayers, he, as it were, throws out a rope by which he might draw them back to himself when the sacrifice was over.