Is it possible to store money in a smart contract where the value of it won’t change?

I'm working on a part of an app where client 1 sends $X to client 2, which will only be released after something has happened. This will likely be a somewhat long term transaction, (a couple days to a couple months) so I need the money sent to remain the same. I know I can store Ethereum in a smart contract, but I don't know how else. Anyone have any ideas about how I can go about this?