Problem
You need to add a credit to a customer's profile for future invoices due to a mistake made in a previous year.
Solution
To add credit to a customer profile, collect a payment from the customer profile and save the payment without charging.
Add credit to a customer profile
Search for and open the customer record you want to add a credit to.
On the customer profile, click More
> Collect Payment. 
On the Collect and Apply Payments screen that opens, enter the details of the credit.
Select Apply to Customer Profile.
Depending on the payment type you selected, click Save or click Charge > Save without Charging.
This process adds a credit to the customer's profile. You can use this credit for future invoices.