Unfairly treated

I've made a booking with the establishment on the 11 July for check in on the 25 November and check out 28 November, I've paid in deposit of R1500.00, due to unforseen circumstances, I've canceled the dates but requested the establishment not to refund me I'll change the dates. It was agreed, now that I have new dates, the establishment is fully booked for my dates, they refuse to refund me they say they keep my money for whenever I want to visit. This is unfair, o have a right to cancel and be refunded if they don't have availability for my dates.

Please assist me