Or go round and 4x2 himJust issue another invoice for the balance of the agreed price. See what he's made of.
My ex boss, some years ago, invoiced a farmer for a trailer, which was paid in full. On going through the accounts at the end of financial year, some many months later, he saw the amount on the ledger and thought 'that's a lot for that trailer'. On investigating the invoice he found that he'd mistakenly overcharged by £1000. It was only a slip on one number of a sequence. Went to see the farmer with a cheque for the £1000+VAT next day.