Hmmmm. Well, "aquarium salt" isn't what makes brackish water, it's made from "
marine salt" like Instant Ocean, Reef Crystals, etc.
I bet, since he has evidently been in freshwater, he will be ok just overnight.
But... that guy at the LFS should better inform himself about the salinity needs of mudskippers, cos that little guy will perish back at the store and so will future mudskippers there
.
Sorry. What a bummer for you. They really seem like neat little fellas, it would be a neat "wet pet" to have with the right conditions.