John Calvin Commentary Exodus 8:15

"But when Pharaoh saw that there was respite, he hardened his heart, and hearkened not unto them, as Jehovah had spoken." — Exodus 8:15 (ASV)

But when Pharaoh saw. From this, it appears that the wretched tyrant, like a winding serpent, twisted and turned his mind to devious plans. For when he was trembling under the immediate sense of God’s power, he dared not obstinately resist any longer; he only sought a little breathing room. Now, being freed from fear, he returns to his former stubbornness.

But this is a sign of a perverse and corrupt disposition: not to submit willingly, but to pay only temporary deference when necessity is unusually pressing.

God foreknew, and had foretold to Moses, that this treachery was hidden in the recesses of his heart. But God was willing to bring it to light and therefore remitted the punishment; and from this came the opportunity for his deception.