But I will indurate his heart, and will multiply my signs and wonders in the Land of Ægypt, and he will not hear you: and I will put in my hand upon Ægypt, and will bring forth my army and people the children of Israel out of the Land of Ægypt, by very great judgments. And the Ægyptians shall know that I am the Lord, which have stretched forth my hand upon Ægypt, and have brought forth the children of Israel out of the midst of them.
Exodus 7:3-4 Original Douai
Indurate I read is another word for harden. Also, make callous, obdurate, unfeeling.
It does not seem to me that by making a man more stubborn God takes away the man's freedom. Freedom is in the will. Our emotions and tendencies are separate matters. They are separate parts of us. While we may have an extreme desire to do one thing or another, the will is still free to choose one thing or another.
Certainly God knows what Pharaoh will do, all the more stubborn. But this hardly matters. God knows all our actions and this does not take away freedom of will. So too often enough we know what another person will do in response to something we do, but this does not take it away.