Fair tips

After curling one night a group of us went to a pub and grub. One of my buddies, I'll call him Cloyce just to give him a name, left early. He left his credit card receipt on the table after he signed it, adding a six dollar tip for the waitress.
It was a fair tip, to be honest. But not fair enough. I added a one before the six.
At 7:30 the next morning I got a call from Cloyce. He monitors his credit and debit cards religiously. "Did you mess with my credit card?" he asked.
"Now why would I do that?" I responded innocently.
"You did, didn't you? You expletive."
Hey, I was going to give him his money back. But he called me an expletive, so expletive that.