I don't think it's a "money issue" that would cause problems in a potential Karlsson trade as much as it would be that San Jose isn't getting compensated enough for the amount of retention they'd have to do.
It's why I think that if Karlsson doesn't get traded this off-season, he's not going to get traded for multiple years. Karlsson doesn't have enough value on his own to make up for the extreme retention that would be required for another team to trade for him. Retaining $4 million over 4 years ends up being $14 million in real dollars with how much money Karlsson has left on his deal, but Karlsson doesn't have enough value on his own to make up for that cost.
If the deal is Petry, Smith, Poulin, a 1st and 2nd for Karlsson at $7.5 million, I don't think there is enough value in there for San Jose to rationalize eating that much money on Karlsson's deal. But at the same time, there is zero chance that any team pays more than that right now. The situation seems to be that San Jose is either going to make a bad deal or they'll keep him, it doesn't seem possible for San Jose to get a great return for Karlsson.
If they can find a third team to eat $2 million while San Jose eats $2 million, I think it becomes much more doable. But that's just going to result in San Jose getting back less value and needing to divert value to that third team to pay them to eat some of Karlsson's deal. At that point, it might be something like:
Penguins get Karlsson at $7.5 million
San Jose gets Petry, Smith, Poulin and a 2nd while retaining $2 million on Karlsson
Team C gets a 1st while retaining $2 million on Karlsson