Is there a hadith stating that the Prophet, peace and blessings be upon him, said to a man who hit his head against a wall as a punishment for looking at a woman, that this was the recompense for his gaze? And is ghusl valid if the person performing it forgets to wash his foot?

Yes, several authentic hadiths with similar meanings have been reported regarding this. Among them is what Imam Ahmad and Ibn Hibban narrated: a man met a harlot in the جاهلية (pre-Islamic era) and extended his hand to her. She said: "Stop, for Allah has removed the جاهلية and brought Islam." So the man turned away, and his face hit a wall, injuring him. When he informed the Prophet, peace and blessings of Allah be upon him, he said: "When Allah desires good for a servant, He hastens the punishment for his sin. And when He desires evil for a servant, He withholds the punishment for his sin until he meets Him on the Day of Resurrection, as if he were a wild ass (meaning, laden with sins)."

And among them is what Abu Nu'aim narrated: a man came to the Prophet, peace and blessings of Allah be upon him, with blood streaming from his face and said: "O Messenger of Allah, I passed by and looked at a woman, and I followed her with my gaze, so my face hit a wall." The Messenger of Allah, peace and blessings of Allah be upon him, said: "When Allah desires good for a servant, He hastens the punishment for his sin. And when He desires evil for a servant, He delays his punishment until he meets Him on the Day of Resurrection, as if he were a wild ass."

Other narrations with similar meanings have been reported from Ammar ibn Yasir and Abu Tamimah Al-Hujaymi, indicating that if Allah the Almighty desires good for His servant, He hastens the punishment for his sin in this world. And if He desires otherwise, He delays the punishment for his sins until he meets Him on the Day of Resurrection.