Write off unpaid portion of customer opening balance

Write off unpaid portion of customer opening balance

We have a client who had an opening balance when we migrated to Zoho Books almost two years ago, but has never been invoiced in Zoho Books. A partial payment was made last year toward that opening balance, and now we would like to write off the remainder of the opening balance. How can we do this when there's no corresponding Zoho Invoice, but rather a customer opening balance?

Thanks for any suggestions!