if a client has some credit avaible, you may pay an invoice with that amount. You should od the following on the device
Main menu > Database
Touch Sales - Invoices
Touch Unpaid invoices-Orders
Search or select for the inoice to be paid
Touch on Payment method > Cash
Scroll up the window to search for Credit Available select then Done
Touch Continue to complete the payment > Continue
Touch Pay or complete payment to finish process