How to Apply a Credit to an Open Invoice
From the Sales → Receive Payments page Select the account you would like to receive payment for.
Once you select the customer, it will populate the grid underneath with all open invoices for that customer.
Once you have your customer selected, select an invoice(s) you would like to receive payments for.
Note: Make sure you select at least one invoice, otherwise you will create additional credit for your customer. See below.
Once you have at least once invoice selected, choose "Credit" from the payment type drop down. You can see the fields will display the credit amount and the total amount remaining after the credit is applied ($120). Click "Make Payment"
After the payment has been applied via credit, the invoice will be marked as Partially Paid and display the remaining amount. The credit option from the Payment Type drop down will also show $0 remaining in credit.