I'm super skeptical anyone would want to touch Letang's deal. Letang still has 3 years left on his deal after this year and he has taken a significant step back from where he was in the past. I think he's probably more of a 3/4 that's just playing way too much on a very bad team at this point, but that with 3 years at $6.1 million is a lot for any team to swallow.
Ultimately I think the Penguins trade Karlsson and keep Letang, mostly because that's what the market will force them to do. They can find a suitor for Karlsson and likely get a 1st for him, just like Seth Jones did yesterday. But Letang's probably immovable due to his contract, unless he has a big bounce back season next year.