Here's a quick tip for Zoho Invoice users who use Google Checkout to receive online payments.
For payments received via Google Checkout, if you were to refund an amount to the customer from the payment amount, Zoho Invoice will capture this refund and update it automatically. For example: If John Smith your customer pays you $100 for an invoice. You refund $10 to John as a discount via Google Checkout. When you sign in to your Zoho Invoice account, you'll see that the $10 refunded to John will be updated under the payment details for John's invoice. The amount received from John for that invoice will show up as $90 which is the payment received minus the refunded amount.
The catch here would be that, the refunds are captured only when the refund is done 30 days from the date of payment.
Zoho Invoice FAQ