Down payment is saved as part of the configuration; you will not be asked to set it during the creation of the invoice. If you would like to change it, go to Sales | Configuration | Settings, find the Deposit product option, change the product to Down payment, and click on the APPLY button to save the settings:
The purpose of adding this line is to keep track of the advance on screen and on the printed sales order. The same line will appear on the printed report.
There will be two invoices linked to the sales order, and you will be shown the list of invoices attached to the order:
The final invoice is created, having two lines--[RAKLGW1000] Laguna Series - Size of 1000 x 1000 mm, with the total amount, and Down payment deducted from the invoice amount. The total of all the invoices raised against the order is the same as the order value.