Is a Muslim sinful if he gives an alcoholic drink as a gift to a non-Muslim, after he had received it as a gift and only later became aware of the presence of alcohol in it?

There is no sin in accepting the gift due to not knowing that it contained alcohol. However, it is not permissible to give alcohol to a disbeliever (kafir), and it is obligatory to pour it out or dispose of it; because the disbeliever is addressed by the branches of Islamic law, and gifting him alcohol is forbidden because it involves aiding in sin and transgression. One must repent to Allah Almighty and not return to such an act.