Poison the student or bribe other students to make false accusations against him, eg he talked about joining ISIS or committing a terrorist attack on the US soil.

I can't believe students now a days man. They have seriously hit rock bottom. I am extremely disappointed.

That sucks, OP. If this student has done this to you it likely isn't the first time. Hopefully his chutzpah will backfire on him and the investigation will out him and exonerate you. In the meantime, be proactive and reach out to other teachers to see if he has done anything like this with anyone else. Also point out to the dean that you've (presumably) had to give bad grades to plenty of other students over the years and none of them have attempted to bribe you - or gone to the dean and told him you were trying to extort money.

Never give a slime bag fair warning that they cod be held accountable, they will always become defensive

"Are you sure there isn't ANYTHING, that I can do, to change my grade?"

This needs a follow up

I hope that kid gets caught and expelled.

That wretched little shit

no good deed goes unpunished, op. sorry but ydi for trusting someone willing to bribe a teacher. did you actually think that would be the end of it?

As a future teacher myself, I have thought many times about this type of thing. I think I am going to record any 1 on 1 conversations I have with students. Just as a safety precaution