Very poor service

We paid for a cruise that was supposed to take of in April unfortunately Corona Pandemic happened so the cruise was cancelled. We understood due to the pandemic and we waited for futher communication which was indeed sent to us that they will give us vouchers to cruise on the upcoming cruise events 2020/2021. The cruise was for 6nights by the way. We futher more received communication that we must choose whether to go in December 2020 or March 2021, we chose Dec 2020. They came back to us told us they will cruise us in December 2020 and we were so happy. Two days after they come back and say we must pay R70 000 haibo how because they promised us that they will give us vouchers to use on the next cruise 50dollara/100 dollars. So we ask how must we now pay, they telling us because it's December I mean we didn't cancel or postpone because we wanted to but only because of the pandemic now they telling us we will sail next year March how when we chose December and also the number of day are lesser in March as it's only 3nights. When we ask for refund they telling it's not an option haibo they don't want to give us our money back and they giving us a cruise that is lesser than what we paid for..not totally unfair I paid about 20k for this cruise I want my money back if they not giving us the December cruise. By the way we booked as group.