I have a feeling the return (prospect and conditional pick - knowing the Oilers don't exactly have tons of high-end prospects) will be less than some are expecting if they just look at Walman's counting stats and usage on a team absolutely bereft of capable NHL blueliners.
There's definitely a world where a rental Dumoulin gets more back than Walman, and people are going to be outraged by it based on "stats" - but a lot of it comes down to the type of defenseman teams are looking to add at the deadline and also that often rentals will get higher prices than guys with term when it comes to depth players. I mean, Soucy still has term and got less than Dumoulin as well. Sometimes the "no strings short term fix" is more appealing.
Something like a mid-prospect and a conditional 3rd that maybe turns to a 2nd (don't kill me, not looking at the Oilers draft pick chart here) seems like it might be in the cards.