I already discussed this via chat and email with Zoho support but was told this is not possible. They did indicate this is doable but only with recurring invoices.

My situation: client mistakenly sent two checks for the same job. He told us to go ahead and deposit both checks and to issue him a credit on his account that shall be applied towards his next invoice. We invoice this client once per year so it's likely we will forget to apply the credit.

Solution: Zoho should add functionality so we get the option to apply OR at least remind my bookkeeper that we owe credit to the client before the next invoice is sent out.