Is it permissible for us to return a car that my father bought in good working order, and which then broke down after only a few days of use, knowing that the seller did not intend to deceive and was not aware of its defects?

If it is proven that the defect pre-existed the sales contract and was discovered after examination, then the seller is liable for it, whether he knew about it or not. The buyer then has the option between retaining the sold item or rescinding the contract upon discovering a defect he was unaware of. However, if the defect occurred spontaneously or due to the buyer's action, then there is no liability on the seller.