The declined transaction is appearing on your statement because the merchant hasn't yet sent us a message to say that the transaction failed, so Curve unknowingly processed the payment anyway. Although this is bad merchant behaviour, it happens occasionally, and does not mean that you will lose your money.
In these circumstances, after 10 days the authorisation will expire, automatically leading our system to issue a reversal. You should see this amount refunded to your account 2-3 working days after this reversal is issued.
However, if you'd rather not wait, please get in touch in-app or via firstname.lastname@example.org with evidence of the decline and we will refund the transaction.