I think it would be prudent to issue an invoice, and on the invoice reference your verbal agreement of x date, and the terms and conditions of your verbal agreement.
Give him 30 days to pay, and mention late payments will accrue interest at 2% compounded monthly. You might consider sending it via registered mail or something so you have proof it was delivered to him.
That way, if it does go to Small Claims, at least you have something because they will surely ask if you sent him an invoice. I'm not sure taping conversations will help, unless of course he knows you are doing it.
Good advice. I might add that if payment is not made, then apart from interest, you will add the costs of collections and attorney fees. (If there was not agreement on interest, at least you can ask for pre-judgment interest according to court rates but I'd settle without the interest).
You can record conversations one-on-one only. It is only when 3rd parties can listen in that you must give notice or get permission.