There is a mechanism to handle the scenario where a debit request exceeding the remaining balance of the card that has been inherited from the mechanism traditionally used by PDQ terminals. This can potentially happen with Redeem transactions.
When such a scenario occurs, the entire remaining balance is debited from the card, and this revised amount is returned to the caller. In these cases, the response will contain the response code 30 which indicates insufficient funds.
It is up to the integrator/merchant to decide whether or not this is desirable (and they may offer the choice to the cardholder). If it is desirable, then the debit transaction has been completed, albeit for a smaller amount than originally intended. It is up to the merchant to collect any outstanding amount by an alternative means of payment. If the part payment scenario is not desirable, then the debit transaction should be reversed or voided.
Important: If you receive a response code 30, the transaction has completed, but for an amount different to the amount originally requested. It is your responsibility to ensure your software handles this scenario correctly. Savvy are not responsible for implementations where response code 30 is not handled correctly.