While we're talking about currency, why hasn't anybody in the US(to my knowledge) made a payment processor that works by generating transaction numbers, and then sending those numbers instead of sending people payment authentication credentials?

Let's say George wants to give Sally $5.

1. George logs into his PayCo. account. He clicks generate transaction, amount $5. The server returns a very long randomly generated key. 2. He copy-pastes that key into Sally's pay-info server. 3. Sally sends that key to PayCo.'s server, money changes hands, and the key is no longer valid.