For the 9000th time, you cannot trade a player into a buyout in order to re-sign him cheaply. Bettman banned that after the whole Orpik ordeal.

https://www.nbcsports.com/washington/capitals/caps-cleared-any-wrongdoing-after-nhl-investigates-brooks-orpiks-trade-buyout-and-re#:~:text=Washington%20traded%20Orpik%20and%20his,%244.5%20million%20in%20cap%20space.

False. The idea was not banned and in fact, one year later the Calgary Flames bought out Michael Stone and resigned him a deal with a lower cap hit. The premise of the whole thing is very much so legal and an option open to the Oilers at this time.

Buffalo says No

They cleared Washington of wrong doing back then, because it was not explicitly stated and their were no talks of a buyout in the trade negotiations with Colorado. They said they will be making an example the next opportunity they get.

https://www.sportsnet.ca/hockey/nhl/31-thoughts-examining-erik-karlsson-trade-fallout/
#14

"Teams viewed the memo as a warning, that if the NHL could make an example of someone, it would."

Stone did not involve a trade and is a separate cap circumvention issue that is now also banned.
The Oilers will get punished