So...apparently NPCs only take exact change.

[Escalon]

Well, I went back, gave him ten gold, nothing. I had some spare pelts so I experimented. I gave him just the medium pelt, then just 15 gold. Nothing. Gave him 15 gold and a medium pelt at the same time, and it worked just fine. Figured I'd check just to be sure. I'll recoup the money eventually. So it indeed appears you have to give NPCs exactly precisely what they ask for, how they ask for it. Now I know.