The Fascists were very interested in getting a hold of Petsamo nickel, which was why the Soviets wanted to complicate access to it:

The Soviet Union had indicated an interest in the Petsamo area during the negotiations with Finland in the fall of 1939, but primarily on the basis of military considerations. When the Finnish–Soviet “negotiations” were resumed in March 1940, Molotov again told the Finns that Soviet military circles wished to see the area annexed to the Soviet Union.

Nevertheless, the Petsamo area stayed with Finland under the terms of the Treaty of Moscow, and there was no hint of any Soviet economic interest in it. The [Fascist] conquest of Norway shortly afterward obviously caused the Soviet leaders to regret their generosity, however.

[The Third Reich’s] interest in Finnish nickel also dated back before the Winter War. The Finnish–German trade agreement of October 1939 envisaged the early exploitation of the nickel resources at Nivala in Qulu province, and the nickel to be extracted was to be sold to [the Third Reich]. In return, the [Fascists] were to deliver, among other items, 134 anti‐aircraft guns, and 50 of these actually reached Finland before the war began.

See here for more.

I did not know the Soviet union offered east karelia