Since this is an API, that means you just build this into your app. You could initially decline the transaction, send a push alert to the user's app notifying them that the transaction is in review, then update them on that status when approved/declined. At that point the card can be run again and approved if that was the decision. You could even allow a pre-request through the app, when when approved would enable your user's card for the purchase. The possibilities are endless.