I’m not sure copper oxidation of the contacts is a warranty issue.
I thought the contacts were gold plated and protected for sometime, and if used regularly should be self cleaning.
The issue may be that you got the phone damp, which would automatically void the warranty .
Just wondering, do you live near the sea etc. Salty air can be an issue.
However you may want to check the dampness indicators in the phone as if Fairphone discover one after you’ve sent it . . . well they may fix it at a cost but that doesn’t mean another part may not fail soon.
Check out this post for pictures of the indicators.
See this and the post two above for how white they ‘should’ be