Since some advisers are assuming a little beyond the actual situation (my fault for not describing what's going on), I'll give a quick rundown on where we're at.
Warning! Potential spoilers ahead!
I'm running a module and the party are deep into it. Probably about halfway through. They've just been funneled into a dragon's lair (two young, white dragons). Just as they were putting the finishing touches on one of them and turning their undivided attention to the second of the pair, a cloud giant, a frost giant, four ogres, and an Oni come at them from behind.
During the fight the party wizard gets knocked out and is laying unconscious. Everybody else is in toe to toe melee, pretty much, and there's the Oni, invisible, flying around, not twenty feet away from the wizard who has a handy haversack on his back and a staff of frost laying next to him.
Long story short, the Oni sees which way the wind is blowing (the 2nd dragon is dead, the cloud giant is dead, and the frost giant is barely hanging in there, while the remaining PC's still have quite a bit of fight left in them), so he snaps up the haversack and staff and off he flies.
His escape is not a clean getaway, but get away he eventually does to a spot further down an unexplored (by the PC's) tunnel where a few of his ogre brethren are hanging back. The PC's are in no shape to go charging down the tunnel after him (they don't even actually know he's down there, for that matter; currently they assume he went the opposite direction towards the dungeon entrance and the great wide open), so there's a few moments of downtime where the party tend to their wounds and check out the dragon horde, and the Oni rummages through the haversack, finding what he assumes is a spell book. He's smart enough not to just open it without precautions, but quickly hatches a plan. He got a decent look at the dragon horde, and thinks if he plays this thing right he might just get most of it for himself.
He knows the party is hurt and in need of rest, so he's got to approach them asap before they get back to full strength. His plan is to write a quick note suggesting a parlay and send it to the PC's via a white flag waving ogre lackey. He'll have one of the PC's meet him at a spot where several tunnels intersect. He'll be invisible and won't have the stolen items with him. One of his ogres has the stash with instructions to destroy it all if things go sideways.
He'll ask for 10k (he figures there's at least that in the horde, and in return he'll give up the book. He'll probably keep the haversack to carry his new loot in, but he might be persuaded to give up the staff as well. Furthermore, he might offer up some info on the main bunch of bad guys who are further in the dungeon, planning a pretty mean ambush for the interloping PC's.
If he can get away with it, this no name monster from the module will suddenly become a whole lot more interesting, and I can see him making appearances in the future. Certainly the PC's will want their revenge. But even now I'm thinking of all kinds of machinations this Oni could be up to. He knows he's pissing off at least a powerful wizard, so it stands to reason he has some back up of his own in the form of powerful friends. Hell, maybe he's actually a mercenary good/bad guy that the PC's will discover, to their dismay, is untouchable. I don't really know at this point, but the possibilities are endless.
So yeah, it's a dangerous thing to steal from a wizard, but he must be fairly certain he can get away with it.
As far as his demands, there's a few other goods in the haversack that he'll probably keep as well, and while the loss of 10k will sting the party a little, it's not the end of the world. Mere gold may seem innocuous, but I figure I can come up with a good reason for him to want straight cash.
I probably won't have him ask the party to quest for him. 1) he knows if he lets them get back to full strength they'd just as likely gear up to deal with him in a violent and murderous manner, and do just that. He want's to get the deal done and get gone while the getting is good. 2) Not every encounter has to be a side quest. Too much of that gets old. Besides, the party already has a laundry list of quests just waiting for them. Trust me, they're plenty busy. I'm actually a little worried that they won't even get to all of them before level 20.
So that's it, in a nutshell.
Oh, and I should probably mention, during the battle I more or less played the Oni straight out of the book, and I'm not inclined to give him additional powers now. For one thing, if he was an arcane caster proper, he probably wouldn't consider giving up the spell book. For another, if he suddenly starts pulling rabbits out of his hat that he didn't while the PC's were trying to chase him down, they're going to find it awfully suspicious and figure out that I'm padding the guy. Besides, Oni's are pretty hardcore as written.
Anyway, thanks for all of the replies and advice. Some good stuff to consider here. And I think ultimately, letting the story and situation decide the ransom price is the right way to go.
