If the credit amount is less than the invoiced amount, the credit is calculated from the same invoice.

For example, your customer purchased a product 'CloudStream' with a base price $100 for 6 months. Therefore each Billing Schedule would carry an amount of $100 spread across 6 months. You've invoiced the customer for $600 in advance.

Now due to a price downgrade ($10) effective from the 3rd month you have to amend the product price. Post-amendment, the existing Billing Schedules are revised to $90 from 3rd month onward. On comparing the old and new Billing Schedules, you can deduce that $40 needs to be credited back to the customer's account for the last 4 months owing to the downgrade. So you must issue a Credit Memo document which has a total amount of $40.