How legally binding are e-mails?

Regardless of the legality of the e-mail, you have a moral obligation to pay this man back his money. After all, you both agree you owe it. Just imagine for a minute if the boot was on the other foot. Ask yourself what you would do if he owed you the money. You'd move heaven and earth to get it back. Do the moral thing, and pay up. Just make sure you get a receipt, though.