In the US, you can contact the venue, and "turn in" the tickets so that the venue can resell. Nothing given to you the donor, except perhaps a tax deduction for the ticket price. (unless you are a series subscriber and eligible for exchanges)
No idea if contacting a venue in these situations and returning the tickets to them is common in Spain. I have known people who stood outside venues and attempted to "scalp" unneeded tickets. I suppose that is workable only if a concert is popular and sold out - and I wouldn't attempt it without knowing the local laws!!