This is a scam. Refuse all requests to cancel. Simply say:
I am very sorry that you are not 100% happy with your order. However, I have completed your order to the best of my ability and to your specifications. To do so, I have had to allocate time and resources to your project, and refuse other work opportunities.
Fiverr terms of service support my right to decline your request to cancel. I will, therefore, not be canceling your order. As my rates on Fiverr are already discounted, neither can I agree to accept partial payment.