If you think about this in minimal terms, you have a merchant(pos), a purchaser/consumer, and an intermediary (though this would not be necessary for direct non-credit payments, like non-escrowed bitcoin for example). In an ideal world, a pos could publish a message like "transaction #N initiated for $X" to some "payment bus". Then the consumer would have to then publish a message like "User $Me agrees to pay $X for transaction #N".

If this were something like bitcoin, the payment could be wallet to wallet. If it were credit, some processor would have to know that they process transaction for "$Me" and find transaction #N to settle it back with the POS.

So the question really comes down to how you would implement this "payment bus". It seems like it could theoretically be something like the stock market. In fact, credit companies could be market makers and offering to settle transactions for consumers at different rates. This would have to be fast enough to be usable, but it seems like that issue is solvable with HFT solutions. But how would you get existing companies who make their own custom and profitable buses to all agree to use one generic bus and move the money making to the credit side?