Partial purchase

There are purchase scenarios where the final amount is not yet know during the authorization. Some examples:

• A vending machine authorizing the value of the most expensive product and then capturing the correct value once a specific product has been selected.
• A gas station pump authorizing a maximum value and then capturing the correct value once the gasoline has been dispensed.

In these cases a higher amount needs to be authorized than will eventually be captured and the client needs to be able to specify what the correct amount is after the authorization. That is done as part of the confirmation step by specifying a captured amount that must be lower than the requested amount. The captured amount can also be zero, in which case the transaction is voided.