John Gill Commentary Exodus 9:35

"And the heart of Pharaoh was hardened, and he did not let the children of Israel go, as Jehovah had spoken by Moses." — Exodus 9:35 (ASV)

And the heart of Pharaoh was hardened
Instead of being softened, as it seemed to be when under the plague, it became harder and harder when delivered from it:

neither would he let the children of Israel go ;
though he had so absolutely promised it, and assured them that he would not keep them, and that they should not stay any longer:

as the Lord had spoken by Moses ;
that so his heart would be hardened until the signs and wonders were multiplied upon him, God designed to perform, (Exodus 4:21) (7:3) .