My BF has teased me over the years about asking for the money. I tell him we can agree to disagree.He would try that only /once/ with me, it would be shut down with a cold reply about his daughter's manners. I really have no patience for that sort of thing.
IMO, hypothetically, even if you had very clearly stated "I do not want this card. I give it to you" I still think she should have realized that you were only giving it to her because you thought that the card was worthless to you. Once she realized it could have been redeemed, I think the value of the card changed. It was no longer worthless. I think any honorable person would have said to you "I am going to see if I can get cash back for the card. Are you ok with that?" At which point she should have offered you the cash back. And, had she been that courteous, you might have let her keep the cash, who knows.
Her behavior just seems dishonorable. It's kind of like receiving a used item from a friend, like an old jacket, and finding cash in the pocket. Perhaps you can say "finders keepers" but an honest person, who appreciates the gift would say to the giver "Oh, I found some money for you. Would you like it?" not keep it for themselves and then flaunt the new cash
in the giver's face like "ha ha. Neener Neener. You were too stupid to keep this money. Look what I just got from you. Sucker!"
Legally, yeah, maybe it could have been hers because she might have thought the card was a gift. But a thoughtful, considerate, appreciative person would not have behaved as she did.