I agree with you in theory, but retaining half is the same as selling Karlsson for a 2nd and 8.75M in cash. He is of no use to them in a rebuild, and with his name in trade rumours he could border on a distraction. So Pittsburgh gets value just by dumping part of the contract.
They also likely want to move him before his 5M signing bonus is paid (likely on July 1st, but some players have different dates).
His NMC and his term at his age is going to make it difficult for them to build up a market.
I doubt they retain half, but a soft return wouldn't surprise me given the situation. What they originally paid for him was a projected late 1st, and they got a 3rd back, and San Jose took multiple bad contracts. That was after e Norris winning season.