To add a gift voucher purchase to a debtor’s account,  

Recommended procedure for adding a gift voucher is to add a new line in the Line Items tab of the Debtor record. Set the Category field to "Payment". Enter the amount in the Amount field as a negative number. Leave the Date Billed field empty; this will cause the line item to be automatically added to the next invoice produced for the debtor. 

Then, in the Payments tab, add further details to the line item. Enter a date in the Date Paid field. Change the Amount field to the amount paid as a positive number. The Method field and Notes field may also be set as required. 

The payment has now been applied to the debtors account balance, and is set to automatically appear on the next invoice produced.