Is a man considered to have sinned if he tries to recover a car he registered in his ex-wife's name, for which she did not pay, and he impounded it and prevented its sale, and requested its registration for the children, knowing that she gave him gold equivalent to one-fifth of the car's price after it was registered in her name?

If you gifted the car to your wife without any consideration, and she took possession of it, then you do not have the right to revoke it. This is based on the Prophet's (peace be upon him) saying: "It is not permissible for a Muslim man to give a gift and then revoke it, except for a father regarding what he gives to his child." And his (peace be upon him) saying: "The one who revokes his gift is like a dog that vomits and then returns to its vomit." However, if the gift was in exchange for the continuation of marital life, then you have the right to revoke it. Similarly, if your wife gave you gold in exchange for your gift of the car to her, then she has the right to revoke it.