What to do?

My kid brought me a progress report this morning. Said he drew on it and scribbled it out. It was on the signature line and just below. It was my initials. I could still see it through the scribbles. :( He says its not. YOU CAN SEE IT!!!!!!!! Grr. I am so mad. Its stupid cause all of his grades are good. 90+ His average is 99. There is a dance tonight and his friend is supposed to come over. I dont want to tell his friend he cant come now because he wont have a chance to make other plans for the dance and his parents may have plans now that he isnt going to be home. Any ideas for a creative punishment? For trying to sign the paper and for the lie.....I dont want to go over board but I want him to know this is not acceptable.