If you see a "pending" card transaction this means the merchant either hasn't fully authorized the payment or it hasn't completed the payment failure yet. What they've done instead is 'pre-authorized' - indicating to us that they have started a transaction and have requested the money for said transaction be held pending authorization or cancelation on their end.
Generally speaking a transaction will move from pending to canceled or posted when the merchant reconciles their books. This usually happens 1-3 business days from the initial transaction date, though it can sometimes be immediate. However, we give merchants an industry-standard 30 business days to process or cancel the payment before we manually cancel it.
If you're seeing a pending amount that's greater than your purchase, this is a pre-authorization. These charges remain pending for a few business days before they either settle for the correct amount or fall off of the account. You're most likely to see this at merchants such as gas stations, in which a pre-authorization indicates you've given permission for the merchant to charge your card up to a specific amount, which is not necessarily the final total.