If you are selling something to one of your existing customers and they do not have enough money or want to pay with a voucher, you can do a partial payment:
In the menu Financials > Selling you will find the button 'Partial payment'.
A new window will open where you can select the payment method, enter the amount that the customer is paying now and then determine whether an invoice should be printed (choose 'Print receipt') or not ('Without').
Then you can go back to the invoice overview and the invoice will be set to 'paid'.
The invoice will be set to 'partially paid' in the invoice overview.
Next time the customer wants to settle this open invoice, only the remaining amount will be requested.