It gets even better. OP's giving an example where he has a specific amount to spend. Like he knows that there are things worth $10, $5, $1 at the store and he takes all of these in the form of qr codes. But what if you're going to a mall, ready to spend a few hundred USD. Are you going to print 3x 100, 5x 50, 10x10 and then juggle these bills around while the clerk scans? This would have to be the slowest and most annoying way to pay at a store. Only a true nerd would choose it.
Like I said I open also to using a plastic (or other material) card that would work like a credit card with daily limits and approved merchants (identified with public keys). The paper bills would be a backup choice for untrusted merchants or when your phone battery dies, just a primitive way to pay without strings attached.