Yes, the Islanders will be playing a (pre-season) game in Nassau, and the arena operator will pay Nassau a penalty

A 6/15/17 press release from the New York Islanders announced that Islanders To Play Preseason Game At NYCB Live's Nassau Coliseum. That's Sunday, 9/17/17 at 1 pm, which is a family friendly time.

The simultaneous press release, Islanders Announce 2017 Preseason Schedule, noted two preseason home games at Barclays Center and one at Webster Bank Arena in Bridgeport, CT.

As Newsday's Jim Baumbach reported, in Islanders return to Nassau Coliseum for preseason opener vs. Flyers:

It will be the Islanders’ only appearance next season at the team’s former home, which means the arena operator will owe Nassau County $1 million for not meeting a clause that called for two preseason and four regular season games, a spokesman for Nassau Executive Edward Mangano said Thursday.
“They are contractually obligated and the penalty will be fully enforced,” Mangano spokesman Brian Nevin said.

This is a step up from a previous reports in which Mangano wouldn't say that the penalty would be enforced. Operator Nassau Events Center--an arm of Mikhail Prokhorov's Brooklyn Sports & Entertainment--wouldn't comment to Newsday.

The upcoming season could be the third and last of the Islanders at Barclays, since the team and arena can opt out of a 25-year contract--but that doesn't leave a place for the Islanders to play. Despite much chatter about a proposed new arena at Belmont, I still suspect the Barclays lease will be renegotiated.