A transaction authorization can be voided by calling the POST /transactions/{transaction_id}/void API. If the transaction was not yet successfully authorized or was already captured, the operation will fail. Captured transactions can be refunded instead. 
curl -i -X POST "https://api.example.gr4vy.app/transactions/fe26475d-ec3e-4884-9553-f7356683f7f9/void" \
    -H "Authorization: Bearer [JWT_TOKEN]"

Please be aware that issuers might not always honor a void request after 24 hours, even when the payment service used might return a successful response.In these situations, our system and your payment service might mark the transaction as void but the authorization has not been released. In most situations, this authorization will automatically be released after 12 to 28 days, depending on the card used.