Is it permissible to give zakat to a poor person other than the one specified by the owner of the money, after being unable to reach him, or to give it to a needy relative without informing the original zakat payer?

If the zakat payer specifies a person or a particular entity to receive the zakat, it is not permissible to give it to others without their permission. However, if they do not specify, it is permissible to give it to any eligible recipient, and your needy relative has a higher claim.