The discussion in this question seems to indicate that my own tzedakah money is fungible; I can collect coins in a tzedakah box and exchange them for other currency when it's time to make the donation. Is this also true when a third party is involved?
Specifically, suppose some people have given me cash to distribute as tzedakah on an upcoming trip to Israel. Do I pass those specific bills along (which are US currency), because the givers said "this is for tzedakah"? Do I make sure I give at least that much in tzedakah but not worry about the currency involved (maybe I converted to shekels)?
If I do not need to use the specific bills that were given to me, do they have any special status? For example, do I need to set them aside for my own (US-based) tzedakah, or, having fulfilled the mission with different money, can I just put them in my wallet and spend them normally?