Tropical is temperature...Brackish is the salinity of the water...Something could be both tropical(likes warm water) and brackish( needs salt in it's water). Apples and oranges.
There are a few freshwater plants that tolerate brackish H2O. I've never experienced sag in brackish H2O. I suspect that it will survive if the tank is not too heavily salted!
Dwarf sags are fine to use in freshwater tanks. I have kept them for years and currently they are in two of my tanks. In a low-light tank they make a good foreground plant although they can reach heights of 10 inches or more under optimum conditions.
my dwarf sags are my favorite plants. at one point I was doing a lot of searching for brackish plants, for a tank I never went brakish with... and found many sources that said dwarf sag would live in brackish water. mine seems to do best in medium-low light, but spread really rapidly in high light.
I have dwarf sags in my fw tank. they've been there for years and haven't had any problems except that the runners sometimes don't root into the substrate and they end up floating.