Probably Charlemagne. If you'll recall, he put Diamonds in the drain, and that was how his troubles started in the first place. He was also one of the townies in One for the Cutters. He's got a variety of hustles, and this one might be a literal diamond that he's trying to barter with the narrator with or else the kind of diamond he's usually associated with--a packet of drugs (worth 7 grand, perhaps?) At any rate, it makes sense that it'd be wrapped in a dishtowel; Holly will later smile politely and tell people that he was lost in the restaurant raids, so he's a kitchen worker of some sort.