But the money available isn't honestly or true reserves and the price given isn't an accurate one. Its a systemic fraud that completely changes the conditions and costs for one side of the "agreement."
Capitalism isn't really a relevant element here, because the foundation isn't an honest playing field. It's like saying people can voluntarily agree to any contract they like, but the political system is able to arbitrarily change the meaning of words so your agreement obligates you to things you never agreed to.
In that situation, the act of agreement at the beginning has little to do with what is being enforced later. A very analogous situation is occurring with our money.