Percy laughs. "Oh, I don't have the drinking horn. It's been on Sir Alec's belt since he won it from Baron Tomor in a card game. You'll have a job to get it off Sir Alec without him noticing. I'm perfectly willing to provide a distraction right before the duel, or you can try to rob him while he sleeps at the Blue Iris tavern. Either way, I get my horse."
I can't say I care much for Mr. Percy. "Well we all know how horses do tend to wander... After the duel is done to our satisfaction, be in the woods behind the stables at midnight". We'll plan the unpleasant accident later.
#1 and "tell me more about the Blue Iris."
Stab him in the throat, lean in close and whisper, "There's your drinking horn, you bastard."
(DM rolls 1d20+4: 19!) "We all want fairness," says the Hon. Percy. "In the name of blind justice, perhaps I can find a means to convince Sir Alec to look away at the proper moment. By the way, I know Lady Lynette has a fine stable. I'm a bit of a horseflesh enthusiast myself..." Thoughts: "If I can get one of Lynette's Waketon chargers out of this deal, I could pay off my debts. Two, and I'll be rich again."
Mental note: Prepare to let Percy be caught as a horsethief after the fight. "Since Lady Lynette is so eager about that horn - may I have a closer look? I'm curious what is so special about it..."