do what Cassp said. admit it was a mistake on your part and you forgot about the " first gig" discount. clear your conscience and move on.
recently had a phone conversation with a club owner at a place where I played a private party. I could tell he was half in the bag but I tried to explain the difference in my pricing for a privat affair and a restaurant gig. He accused me of being dishonest with him so I told him the price for a 3 hour restaurant gig and hung up.
I was trying to be upfront and I always have done business this way. My point is a clear mind is worth more than a few bucks.
You have a unique talent so things will work out.