Originally Posted by Goldorak

If your PRG-AMS flight was 150 EUR, your voucher will be 150 EUR. If you buy a new ticket at 200 EUR, you will use the 150 EUR voucher and will pay the 50 EUR difference by credit card. If you buy a new ticket at 100 EUR, you will use 100 EUR from your voucher and you will receive a new voucher for the remaining credit of 50 EUR.