There are a number of complexities involved but external factors outside of system's control are most critical. Consider for example a customer that places an order with a merchant for $800 paying for three items with prices of $200, $300 and $300 - merchant tries to settle $300 for item 2 that is available in the inventory, then $200 followed by $300 the next day. What if the original $300 got declined due to insufficient funds (bank transfers can take hours or even a day to decline)? How does the bank know whether the next $300 is a retry or remainder of the original authorization? The correct operation requires cooperation and idempotent behavior from the merchant, their acquirer, the network used by the customer and correct implementation at the issuing or money holding bank. Implementing distributed transactions and settlement ledgers across multiple parties? That's what blockchains are for but we are quite a ways from that (part game theory, part tragedy of the commons, if you will).