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.