If the invoice you are entering is invoiced previously by way of an advance invoice, you can connect the advance invoices to the final invoice you are entering. By connecting an advance invoice to a final invoice the amount due from the final invoice will be reduced by the advance invoice amount. (if the advance invoice is paid) When the invoice is printed the references to the connected advance invoices will be also printed.
An advance invoice should exist.
The advance invoice will be connected to the final invoice and the final invoice information will be adjusted accordingly. (postings, due amount etc.,)