Can I sell my ex’s Boomtown ticket even though he’s paid for it?

Location: England

TLDR; I put a direct debit for 2 boomtown tickets under my card, one has my name on the ticket, the other has my ex’s name on the ticket. Both tickets are paid in full, worth £370 each and he’s technically paid me back for it. Where do I stand with reselling his tickets?

I broke up with my ex when he choked me during an argument while we were on holiday 6 months ago. No one knows the real reason we broke up except my best friend. He’s moved on and in a new relationship (a month after we broke up mind you) and there have been no repercussions. I had a bit of a delulu stage and reached out to try and be friends, but he said in conversation recently that he’s ‘not sure he can take the risk of staying friends’ and repeatedly asks me ‘I need to know if I’m still going to Boomtown’, and it occurred to me he knows I shouldn’t be treating him as nicely as I currently am, and doesn’t want the past of how he treated me to come out.

My ex owed me £700, now down to £350 which was one reason for staying in contact with him. I’ve finally come to my senses and realised there have been no repercussions for the way he’s treated me throughout our relationship where he’s been verbally abusive with regular crash outs and physically abusive in the beginning and again at the time of our break up. I now want to sell or withhold his Boomtown ticket, as I feel it’s the only reason he’s being amicable with me. Context: he’s owed me hundreds of £ for 6 months which he’s been chipping away at, and finally somehow can pay it back in one go since I said I’ll send the ticket once he pays me back everything he owes me.

The direct debit has been coming out of my card for the last 6 months and it’s fully paid off now. I don’t know what the legal repercussions might be as technically he’s paid me back the value of the boomtown ticket. I have currently told him I’ll send the ticket once he pays me back the rest of what he owes me in full (£350) , now I want to tell him I’ll give him the ticket if he tells everyone the real reason why we broke up, which he won’t. This ticket is the only leverage I have at the moment

Maybe I’m evil but just floating ideas out here but I want to sell the ticket, send him a fake QR code so he drives up for no reason and the ticket doesn’t scan, drop that I already sold his ticket, then after the event give him the money back that I received for selling the ticket.

The only thing is he lives locally to me and I’m worried about what he might say about me to mutuals we have, but this is a bridge I will cross depending on what I decide to do.

Alternatively I just collate all the receipts of him admitting how he treated me and forward them to his girlfriend if I can find her socials.

Help please, I’m lowkey spiralling and fuming about the situation, I’m also a very forgiving and kind person but I just need there to be some consequences for once. We did have great moments in our relationship, and he’s since apologised for everything he did wrong throughout our relationship, but I can’t get over him saying ‘I need to be the best version of myself for the woman I love’ speaking about his new girlfriend when all I got were the worst parts of him