I think the barman had decide he was going to pinch one of your drinks anyway, regardless of whether or not you'd picked up the "clean" one. It doesn't sound like he's the kind of bloke you want to call your friend, Mince, even if you're desperate for mates.
Firstly, you should knock him off your Xmas-card list. He's not worth it. Has he got a girl-friend? (Or boy-friend). They should be warned if he's that callous.
It's all very suspicious. I would be so wary. If he said something straight-forward like: "I'll give you Mr Jackson's phone number for ?300", it would be better. At least, however bad the whole thing sounded, you would know what you were agreeing to.
I'm also uncomfortable about a phone number being worth 300 quid. That's got to have "Mr Big" written all over it, or "Writ Large" as trendy folk would have it. I don't want to know Mr Big, 'cos he doesn't represent anything I aspire to.
Mince - you're on your own. You got yourself into this mess, and although I sympathise, I can't help you. Don't phone Mr Jackson. Only an idiot would chuck away ?300 for his phone number, and I wouldn't want it to be you, (in case it reflected badly on this website). Tarquin will back me up on this, I'm sure.