Tim Hortons complaint: Scammed by the server at the drive through.

I ordered a pepperoni pizza and a large London fog through the drive-through. When I got to the drive-through, I gave the gentleman my gift card of $20. The order came to $15.11
He gave me the pizza and the London fog and I waited to get my card back but he didn’t give it back and then I said to him where’s my Tim Hortons card. He said I gave it to you . I said no you didn’t. He said again , yes I did. Made me feel I had a memory lapse. . I said no you didn’t give it back . He said again that he had. I said well it’s not like it’s a lot of money but it was enough for coffee the next morning. I knew he was lying and I couldn’t shake that I just been scammed going through the drive-through. It’s the honesty and integrity of the staff. He didn’t even give me a receipt, but said he did well where is it then? the principal he stole from me and he was really well aware that he did. He had a big smile on his face the whole time that I lost my mind. I realize we’re only talking about $4.89 but it’s the principal he stole.
I was angry inside when I left and I’m still angry this morning. How many times a day does he get away with this? I would like it investigated..