Well, then the solution is simple. You two get together and come to some agreement on how much he's going to pay you for each email that ends up in one of your inboxes. When you receive an email (or many), send him an itemized invoice and he can send you a payment. If there's a problem, just take him to court with the contract in hand and demand payment.
If he had asked me, I would have agreed to $10 per email -- quite a bargain compared to the $100 (plus damages) he agreed to when he decided to spam.