Go to the client's profile page. On the left, under Actions, click Record Payment
Enter your payment amount of 100 Set the payment type to In House Credit
Set the currency the payment should be credited in Do not select any invoices, if any are listed, unless you want to apply the credit to those invoices manually Continue to the next step Submit payment
Go to the client's profile page. Look at the Transactions list, and find the approved transaction. If there are too many, you can instead look for the closed invoice, and click on the invoice row to see a list of transactions applied to it. In this case you would click to edit the transaction. Click Unapply to unapply a payment from the invoice. The transaction amount that was unapplied will be added as a credit. Then you can edit the invoice and Void it.