If you sell gift vouchers then you'll need to account for them properly. There are numerous different ways to do this.
Create and Configure the Sales Type
First go to Settings > Chart of Accounts > Show Advanced Configuration Options >Tick Configure Nominal Codes > Update. Then go to the Sales > Sales Types > Create a new Sales Type. Call it Gift Vouchers and for the Code Type set it as a Current Asset / Liability.
Selling a Voucher
To record a sale of a gift voucher, raise an invoice in the normal way and select Gift Vouchers as the sales type. Alternatively you can just record this in the bank area as Money In and select Gift Vouchers Your Balance Sheet will now show a liability equal to the amount of the Gift Voucher
Redeeming the Gift Voucher
When a customer purchases something from you and pays with a Gift Voucher then you just raise the invoice in the usual way, however you need to also add an extra line to the invoice for the amount of the gift voucher as a negative, so if it's a £20 gift voucher you add a line for -20 and set the Sales Type to Gift Vouchers. This clears the liability from your Balance Sheet and settles the invoice.
Part Voucher / Part Cash
If the invoice is for more than the invoice amount, the negative line you add should be for the full value of the Gift Voucher. This will then leave the amount left to pay in cash as owed on the invoice and you can record this as a regular payment to the invoice. If the invoice was for less than the value of the Gift Voucher then the invoice will show a negative amount owed. This is indicating that you owe the customer the money, rather than them owing you, simply give the customer the cash and record it as a negative payment on the invoice to settle it in full. Alternatively you can just leave the invoice overpaid and the next time you invoice that customer you can use that over-payment to reduce what they owe on the new invoice.