The easiest way is to open the invoice and add the 80$ to the total sum. The trick is to add them with the minus sign. The software will make all the calculations and display the amount left with plus. Save the invoice and that's it. It's usually best to add or keep the records in a database to see exactly what has been added. Additionally, you can print a check or a bill with the remaining amount of money that needs to be paid to you.