Hi MelHarrington,
I see what's happened there. Classic overpayment mix-up, always a bit of a headache. What you'll want to do here in MYOB AccountRight is record a negative invoice for the overpaid amount. That'll let you properly offset the extra payment without needing to refund the whole invoice.
If the payment's already been applied, you can either delete it or reverse it first, then reapply things correctly. If that's not doable (like it's already been reconciled), another option is to create a customer credit against that invoice and use that to balance things out. Here's a guide you can follow for the steps: Customer returns, credits and refunds.
Regards,
Sai