The confusion around this is that in actual dollars, the cap space is about $400k, but when you factor in pro-rating the cap hit based on the number of games remaining, the Pens can add a player whose full-year cap hit is up to about $1.8M without having to unload additional salary in return.
Barring calling anyone up or making any further moves, then each day the full-year equivalent number will go up by a little bit, as the number of days the cap hit is being pro-rated against decreases.
CapFriendly shows that if they wait until the Trade Deadline, that number will rise to a little over $2M.