You missed Bitt's point.

Say you went into a store and bought a $10 CD (yeah, right...) and then the clerk asks you for $11. You hand him a $10 bill and state that he is responsible for sales tax, not you. We know that in the real world that the clerk is going to tell you to either pay up or get on your bike and piss off, but let's pretend for a minute that he didn't. How is he going to convince you to pay? What gives him the right too?
If the cost is $10 and you offer $10, then surely he is (morally) obligated to accept that offer. He gets to keep $9 after paying the 10% tax.
Now if the cost is $11, you'd have to offer $11. He'd get to keep $9.90 after paying the 10% tax.