What is the ruling on the reward (car) I received from a charity after donating to it, knowing that the two Shaykhs Ibn Baz and Al-Qaradawi prohibited rewards on purchases?

It is permissible for the questioner to take the reward, but it is better to relinquish it to a charitable organization so that his good deed may be complete and he may retain his full reward in the Hereafter. This is based on the saying of the Prophet (peace be upon him): "There is no raiding party that raids in the way of Allah and acquires spoils of war but that they hasten to take two-thirds of their reward from the Hereafter, and one-third remains for them. If they do not acquire spoils of war, their reward is complete for them."