SIAs monetary foundation is it's own coin, SIA coin, everything is calculated in that currency.
if the price of the dollar goes up and you use dollar to buy SC, then you are right. But remenber that the price is calculated automatically, so if the net work's host are happy with the same price in dollar as before the rise they could lover their prices in SC keeping it stabile in USD. I think it is still too early to see how this correllates given the very small demand for storage and the in all respect little competitiion there is between the small number of hosts at the moment.
If you quit the allowanse, there is a part of the fund you will loose, but since you then have used no storage, you have no spending on that bit, so you will get most of it back once the contracts expire.
