I think the Oilers would buyout Campbell before retaining half his salary.

The league Says no,

Because you are only allowed 3 retention slots and SJ already used them

So the trade to Pittsburgh can't happen

Sharks say no, because if they trade for Campbell it’s to get good pick(s) and the retention is pointless.
NHL says no to the PIT trade cause the sharks don’t have anymore retention spots.
And Oilers say no cause it’s cheaper to buy out than do that retention.

campbell offers nothing to the penguins.