Parents: how to teach kids not to be selfish with money?

I gave my 11 year old $30 to buy her own food when she goes out with my sister. When they came back from their trip, she still had the money and said she “didn’t hear that I told her that.” I told her to treat her aunt out to dinner next time. Fast forward to next time, and she ended up buying toys with the $30. I don’t want her to grow up being stingy who doesn’t know the value of giving back. Have any of you experienced this? How did you handle it? Thank you.