What is the way to purify the earnings from a taxi bought with money, a quarter of which is unlawful? Is it sufficient to donate a quarter of its price for it to become lawful, or must it be sold, or will its income remain unlawful no matter what is done?

It is obligatory to remove the forbidden amount from money mixed with permissible (halal) earnings to clear one's conscience and purify the remaining wealth. There is no harm in benefiting from a car that was bought with mixed money, as forbidden wealth pertains to one's liability (dhimma) and not to the specific item it was used for. It is not necessary to sell the car; rather, it suffices to remove the forbidden amount from it.