It's not a good outcome, that Ri-Dul respects him a little more, but the other possibility mental states that Ri-Dul could have entered into down this conversational graph were worse in expectation. Some losses are inevitable even if you play your best.
He goes into the next building that looks affluent enough to afford five Wish diamonds and offers Wish purchases, and trades five Wish diamonds for rather a lot of platinum. If they do have to send somebody back to the City of Brass to buy more things, they won't need to pay with Wish diamonds, if somebody suspects those prices are about to drop.