Trump can just whip up his own memos... then it’ll be his memos vs. Comey’s memos... same thing as a “he said she said” situation.
What if: Comey wrote a false CYA memo after the meeting and when he was fired, he immediately gave it to his buddy to leak to the press. After President Trump’s tweet about possible tapes, he attempted to retrieve the memo but found it had already been given to the press. He woke up in a cold sweat when he realized the memo was now evidence that could be shown to be false.