You don't have to trade Pettersson as a rental. You can give him a 5 year deal and trade him in year 3 of it or something like that.
Again, Dumoulin's case is different because he was ruined by injuries. Go look at what Chris Tanev was traded for last year.
If Pettersson gets traded right now, he's probably getting back 2 2nds. If he's traded at age 34 and isn't ruined by injuries, he's probably getting back 2 2nds. If he's traded at age 31 with 3 years at $5.5 million left, he's probably getting back 2 2nds.