low effort unintentional robin hood

I was just out to buy a snack, and in total the cost was 3.50 euros. I gave the clerk (who was new) a 20 euro note and three coins which in total were worth 0.5 euro, so that he could easily pay me back 17 euros. But due to an error of his, he gave me 37 euros. So I returned the 20. Now...