Is it permissible for a Muslim who sold a car with a minor defect, was unaware of its location, and five years have passed since the sale, to donate the value of this defect with the intention of absolving himself?

What you did by concealing the car's defect is forbidden deception, and the Prophet, peace and blessings be upon him, said: "Whoever deceives is not from me." If the seller conceals the defect and uses falsehood, the blessing of his sale will be obliterated, for the Prophet, peace and blessings be upon him, said: "If they are truthful and make clear, they will be blessed in their transaction, and if they lie and conceal, the blessing of their transaction will be obliterated."

You must clear yourself of the deception by searching for the buyer through the traffic department, explaining the matter to him, and returning to him the difference in price between the value of the car without the defect and its value with the defect, and asking for his forgiveness. As for the falsehood, you must seek forgiveness and repent to Allah Almighty.