1. On one invoice, you can log an over payment.
2. Any overpayment on an invoice will not carry over to another invoice. In other words, we do not keep track of the customer's "account balance". Everything happens per invoice.
In short, we don't really support overpayments.
Obviously, this is not a bad problem to have. Paying clients is good in general. So ... just enjoy that mistake while it happens. :)