Is it rude of me to give away a gift?

My (17m) parents just gave me a $30 Starbucks gift card for my birthday. I go often but my little sister (14f) and her friends are obsessed. I asked the girls if either of them wanted to use it up and not say anything. My sisters best friend (14f) agreed to it and took the gift card. I have not told my parents but they literally just gave it to me a few days ago. I know she will spend it up and they won’t know anyway. Was it rude of me?