In certain circumstances, the authorization amount can be different from the settlement amount. This can happen when the final amount of the purchase is not known when the authorization message is sent. The most common examples of this would be restaurants, hotels, car rental agencies and automated fuel dispensers or "pay at the pump".
When the authorization and settlement messages do not match, your bank will proceed to post the settlement amount sent by the merchant to your account without immediately releasing the authorization hold. This means the reduction in available credit remains for a period of time (usually up to five days) before automatically expiring.
When you pay at an automated fuel pump, the merchant may request a pre-set authorization amount. Your bank will put an authorization hold on the requested funds and immediately reduce your available credit equal to this pre-set authorization amount.
For example: your credit limit is $500 and you intend to make a pay at the pump gas purchase. After you have inserted your credit card into the automated fuel dispenser, the merchant may send an authorization request for $100 before you begin filling your tank. Your account is not charged as your total balance owing is zero until the settlement message is sent by the merchant, but your available credit will immediately be reduced by $100 to $400.
If the actual amount of the gas purchased is $50, the merchant will send a settlement message to your financial institution for the final purchase amount and your financial institution will try to match the settlement amount against the authorization amount. If a match cannot be made because the authorization amount and the settlement amount are different, your bank will post the actual purchase amount to your account and the authorization hold of $100 will still remain on your account for up to five days before it is released. Your balance owing is only $50, but for up to five days after your gas purchase, your available credit is $350 because it includes both the actual purchase amount of $50 plus the temporary authorization hold of $100 that was requested by the merchant.