Overcharging on voucher

I have always been happy with Safair but now I feel I am being **********. My flight was cancelled due to COVID and a refund refused by Flight Centre, I was offered a voucher. Now I had no problem with that at the time but now that I want to use the voucher its a ripoff. The price I am charged for a fight from PE to JHB is almost double than the online price and I am told by the travel agent it is because the airline insists that voucher tickets are booked on the same platform it was originally booked on. Why is there such a big price difference? Why must I pay more now, it's not my fault the flights were cancelled. Who is ripping me off? the airline or the travel agent? A voucher should be open to using on any local flight at the current online pricing.