If I promise to give you something, but later say, this other thing is just as good, I reneged. I defaulted on the promise I made.
No. I am giving the currency to purchase the thing. Do you think the US should have paid exorbitant prices for silver in order to allow us to mint silver dollars instead of just giving you a reserve note? That would be incredibly stupid. But that's a disgression. you STILL don't seem to able to answer the question.