Hi Ethel....
You didn't say whether he made an adjustment later or not, but I assume he didn't.
I would take the receipt and be extremely polite and ask to see the owner...who probably wouldn't know anything about this. Just present him with your receipt and tell him what you purchased and that you think that his employee accidently made an error in calculating your fabric.
Being nice and polite gives you that extra upper hand and you might possibly get a credit for the overcharge...doesn't cost anything to try and you just might be a happy shopper on leaving that LQS.
Good luck.....
MJ in NJ