*****ulant with clients

I have sold goods to one of their clients and still haven't been paid after I received email confirming that payment is there and will be released. This morning after I tried to contact them again they send me a mail saying I must first pay R 2000.00 before the funds can be released due to a error from their client for paying in too much money. Since then I did not have any response from them concerning the matter and also haven't received the payment due to me by them.