Failed to give full refund on cancellation

I booked two Business class tickets on Ethiopian Airlines which I was forced to cancel due to illness . The cancellation charge given by their call centre and Web site was $150.00 per ticket or $300.00 for the two . It took four weeks for a refund to be processed at which time I found they had deducted approximately $720.00 or $360.00 per ticket . Repeated requests for an explanation, including emails , and complaints on their web site are simply ignored . The English spoken by their call centre representatives is not good and I struggled to get any response from them . I have been short paid by an amount of $420.00 and would like this refunded .