The $750k difference is small, but its enough to fill another roster spot with a league minimum veteran contract for this season and next if need be. Every dollar counts now. Taking a bad contract back (Nielsen) likely means you don't have to cough up a 2nd round pick, although Isles are in need here and Detroit is not, so maybe you do need to cough up some decent value (who knows), but it shouldn't take a 2nd rounder for a $750k savings.
$750k is some savings, but we need to dump more, and Cizikas is the most likely candidate. He is a good player and fan favorite but I doubt he is an Islander after next season given his UFA status and big contracts down the middle in Barzal - Nelson - Pageau. If you find a suitor that thinks he has positive value, the Islanders should trade him now - to anyone that wants him for whatever they can stomach. Islanders free up $4.1 million in cap space in Year 1, potentially preserve their future 1st/2nd round picks, and have a hopefully serviceable 4C replacement in Nielsen, who is familiar with the Islanders. They can use the 2nds to dump something more moveable (see Komarov/Clutterbuck/Hickey) to free up more space as needed to round off the roster this year and create some room for Pelech/Sorokin/Tito raises next year.