Gregor's father hands him the coin, and as he touches it the surface feels like it's covered with some sort of powder or film. As he rubs it between his fingers the gold sheen smudges off and the coin now appears silver. Someone has cast a very convincing illusion over this coin.

"Do you see what's the matter with it?" the senior Garrelson asks, clearly still fooled by the illusion even as Gregor sees through it. Gregor is unfamiliar with the specifics of how such illusion magic works, but the scam is fairly obvious: to pass off a less valuable coin as a more valuable one. As far as he knows, Ladia Kelstrop has no magical training and couldn't be responsible for this, which means someone else must have paid her with the glamored coin which she in turn used to pay the Garrelsons.