Delay to refund a returned item

I am very disappointed with yellow sub trading,I returned an item because it was big.I paid for the return of the item.I asked for a refund and I was told that the item was on sale,I can only get a voucher.I dii not get tha voucher even today.When I send them emails,they promise to get back to me but they never do.I love buying from them but I am disappointed that they did not handle my returned item issue.