Yes, the charge would have to be reversed in 2021 if by chance the conference was cancelled. But you could easily do that by decreasing business expenses by that amount.
Personally, I'd go ahead and deduct the conference fee.
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
