A customer purchases 100 custom-printed invitations for an upcoming event.
Products get returned.
Services get partially cancelled.
Overcharges happen.
Refund needs to be processed.
When these situations arise, it's important to adjust your records properly without disrupting your accounting history.
Customer details
Product details
Difference in quantity (in case of returned goods)
Price adjustment (in case of a change in price)
Credit Notes can then be applied to outstanding invoices or recorded as a refund, depending on the situation.
A customer returns part of their purchase after receiving damaged items at a retail store.
"How do we adjust the invoice without deleting it?"
Create a credit note for the returned items. The original invoice remains intact while the adjustment is properly documented.
A consulting firm gets a notification from one of its clients not to proceed with one phase of an approved project.

Before creating a credit note, determine what outcome the customer expects.
Ask yourself:
Is the customer returning goods?
Is there a service cancellation?
Was there a pricing mistake?
Has the customer overpaid?
Does the customer want a refund or a future credit?
Capture the reason on the credit note for future references and the answer determines how the credit note should be applied.
An Invoice records what was billed. Credit Notes record what changed afterwards.

Things to Remember
A credit note can be created in two ways:
Create a credit note against an invoice
This is a common approach. If a customer returns goods, cancels part of the service or requires an adjustment on an existing invoice, you can create a credit note directly against that invoice.
This creates a clear relationship between the original transaction and the adjustment.
Create a credit note for a Customer
Sometimes a credit may not be tied to a specific invoice.
For example:
A goodwill credit
A customer compensation credit
A credit carried forward for future business
In these situations, you can create a credit note directly for the customer and apply it later when needed.

Pro tip:
A credit note remains open until the entire credit amount has been utilized. You can close it by applying the credit to one or more invoices, or by recording a refund against the credits.
If you notice old credit notes remaining open for an extended period, review them periodically. They often represent credits that can still be applied or refunds that may still need action.

Watch out:
Zoho Invoice helps you create credit notes and record refunds, but it does not process the refund transaction itself; the payment gateway handles the funds.
If money needs to be returned to the customer, you will need to process the refund through the gateway, bank, or an offline payment method while recording the refund transaction in Zoho Invoice.

Important Links:
Understanding Credit Notes: https://www.zoho.com/in/invoice/help/credit-note/
Closing Credit Notes: https://www.zoho.com/in/invoice/help/credit-note/closing-a-credit-note.html
How to record a refund?: https://www.zoho.com/in/invoice/kb/credits/refund-invoice-amount.html