Should I donate the value of the original headphones or return the gift to my relative, knowing that the headphones are counterfeit and I accepted them as a gift?

Selling counterfeit goods as original is forbidden deception. However, the sale itself is not invalidated by this deception and misrepresentation. There is no harm in you accepting it as a gift, and you are not required to return it to him, nor do you owe any rights to the original company.