Are the proceeds from the sale of a car that a father gifted to his daughter considered property of the heirs, and will the father be held accountable for gifting it to his daughter to the exclusion of the other heirs?

The knowledge of the father's reckoning concerning favoritism in gifting is known only to Allah.

As for the validity of the gift; since the father gifted the car to the questioner and she took possession of it before his death, the gift is valid and the car becomes the property of the questioner. This is the view adopted by the four schools of thought, even the Hanbalis, who believe that if the father dies before rectifying the favoritism, the gift is affirmed.

Nevertheless, some jurists have held that the one who was favored must return what they were favored with to the estate if the father died before rectifying the favoritism.

As for the father's sin due to favoritism, it cannot be definitively asserted. Perhaps he sought a fatwa from a scholar who ruled that favoritism was not obligatory, or he saw a justification for it.

It is better for the questioner to pray for her father's forgiveness. If she wishes to take the more cautious approach, she should return the car to the estate and have it distributed according to Islamic law.