If one of your existing clients (not a Walk-in Client!) does not have enough money or wants to pay with a voucher, you can do a partial payment.
In the menu 'Selling' you will find the button "Partial payment".
Under 'Received' you can enter how much the customer can give and then select the payment method (e.g. voucher). Last you confirm the process by clicking on 'Print receipt', 'Invoice (PDF)' or 'Without', whichever suits you best.
Next time the customer wants to settle his open invoice ,you will directly see in the overview that a partial amount has already been paid.