Hi
What you are talking about in NAV is called "Application" it is when the payment is applied to the invoice and closes the invoice off,
In NAV you have a couple of options,
When you are making the payment on the payment journal you can click on "Functions" -> "Apply Entries" while on the payment line and apply the payment to the invoice there,
After the payment has been received you can go into the vendor ledger entries and select the payment line, then click "Functions" -> "Apply Entries" and apply the payment to the invoice
Lastly there is an option on the vendor card, under the payments tab, called "Application method", setting this to "Applies to oldest" will have the system automatically apply any payments to the oldest invoices, so no manual application is needed
This is not a great option as it i will always pay and close the oldest invoice even if you are actually paying the second oldest, so i would recommend that you rather manually apply
Cheers
Nev