Is it permissible for a woman to demand the return of a sum of money she had gifted to a person for the purchase of a car, after their separation, on the grounds that the gift was not accepted?

What the questioner did is considered acceptance of the gift, and the majority of scholars do not permit revoking a gift after it has been taken possession of, based on the Prophet's (peace and blessings be upon him) saying: "We are not like the evil example; the one who revokes his gift is like a dog that returns to its vomit."

The ruling does not change regardless of whether the gifted item was money that changed in value, or whether the questioner sold his old car and gifted its price to the woman.

Thus, she is not permitted to revoke her gift.