Which is better: slaughtering a sheep or donating its price as a "fidyah" for a new car?

Slaughtering as an act of gratitude for the blessing of purchasing a car is permissible. However, if it is done to ward off the harm of Jinn, then it is not permissible. Donating the price of the slaughtered animal is better if there are poor people in need of money. Otherwise, slaughtering is better due to the feeding of the poor involved, which is among the greatest acts of closeness to Allah.