Take some water out of your tank while you acclimate the fish, keep it in a fish bucket, then put it back when you're done (to allow for displacement of the bag).
Of course, you'll be throwing out some of the tank water when you throw out the acclimation water anyway, so you'll need to make up some water. Once the saltwater is mixed, it keeps for a while, so you could keep a spare bucket of mixed up water on hand, just in case.