1. Select the right Payment method. You can set your payment method to be Auto paid or not, and if not, how many days will be the due date. Click here to learn more about the Payment methods feature

This is what happening is the payment method is set to be Auto-paid. If you create an invoice like this, you don't have anything more to do with the invoice and the partner, because it is fully paid now.

And this is what happens after you set a non-auto-paid Payment method. 

You will able to manage the payments after you save, send or print the invoice

In your invoices page you can easily find the unpaid invoices because they are orange coloured

Just hover your mouse over an unpaid invoice, and click the Payments button, to open the payments page.

After you received a partial or full payment here you can add a payment date and a Document number (optional) and a received amount. Our example we got a payment from $300.000

Now you can see the invoice is still orange because it is still unpaid, but you can also see how much payment is left from the full price