The question is Hashem strengthened Pharaoh’s heart, so where is his free choice? The Dubner Magid
explains that Hashem gave him back his free choice. He originally did not want to release the Bnei Yisroel but the makkos change his mind. He was given back his original will. The Rambam says that sometimes free choice is taken away. Maybe, if Hashem does not help, we cannot overcome our bad inclinations, so Hashem took this help away.
We also see from the Korban Pesach and Milah the concept of free choice. The Jews sacrifice the lamb of the horoscope for Nissan, the idol of mitzryim, the mazal, to show that we are above the stars and they don’t predermine our fate. The Sefer Hachinuch says that Milah reflects this idea indicating that we have the ability to change ourselves. The Tzetil Katan by the Noam Elimelech says that our purpose in this world is to change our nature.