Is it permissible to retract a gift if the donor discovers later that the recipient will use the money to purchase something unlawful, knowing that the donor was not aware of this at the time of giving the gift?

If the gift was completed according to its conditions, with the transfer of ownership and the donee's possession, then the donor bears no responsibility if the donee disposes of it unlawfully, unless the donor had prior knowledge of that. Similarly, the donor is not permitted to revoke the gift after its completion, unless it is a father with his child. Therefore, you are not permitted to revoke the gift, and the sin of purchasing unlawful things with the money falls solely upon the donee.