Trouba may be looking for a huge contract, but that doesn't mean he will get anything close to what he wants. Look at the contract Jones just signed with Columbus, $5.4M x 6 years. I don't see how Trouba can expect to get more than that, certainly not $7-8M over max term. I also think he would take a slight discount to play at home for his childhood team, knowing he will be a staple on the top pair for the duration of his contract. If we were able to trade for Trouba and sign him to around what DeKeyser / Jones just signed it would be huge for this team. I'd go as high as $6M, but I think even that would be a bit of an over payment as of right now.
As for cap space, yes we would have to get rid of a big contract in order to make it work, but regardless of this trade scenario, Holland should be working on freeing up cap anyway. Howard should be gone by the end of this season at the latest. There are always ways to shed cap space.