The superbucks are added right away. Unfortunately, there are cases when payments get declined as late as two weeks after the purchases were made, because of insufficient funds or because of other various reasons.
If the payment was for an item, the system will delete the item automatically, once the payment is declined.
If the payment was for superbucks, the system will try to deduct an amount of superbucks corresponding to the declined payment. If the purchased superbucks have already been spent, the system will deduct whatever superbucks it finds, even if they were received for the mayor's residence, leveling up, daily bonus and so on.