Applying balance from one invoice to the another one

To use existing credit note or overpaid invoice to settle another invoice, you need to go to the accounts invoices. Here you need to find the credit note or the overpaid invoice. Click on the the invoice number, to open it.

On the next screen you can choose "Apply credit to"

NOTE: "Apply credit to" button is visible only if balance of the invoice is negative which means invoice is a credit note or it has been overpaid.

Now you will se a pop up window, like the one pictured below. Here you have to fill in amount and the concerned invoice. Note that amount should be negative as it will be subtracted from the concerned invoice.