I go by today's KJV which doesn't include Tobit, so the first mention of Satan in the garden is the Book of Revelation. If it's mentioned in Tobit, fine, I have no objection and don't see why it matters one way or the other, it's still later than Genesis and makes the same point that when a fact is revealed has nothing to do with the extrabiblical idea of its supposed origin as a concept.
And you are wrong that Satan is not in the OT: I searched "Satan" at Blue Letter Bible and got:
1 Ch 21:1
Job 1:6-7, 12, 2:1-4, 6-7
Psalm 109:6
Zec 3:1-2