Why did the elder think he was in such a conundrum? All he had to do was go "My goodness! I had thought that was all the salt you had! It's why I was so sure I could buy it all, but I had no idea you had even more, that's quite amazing!" and the whole audience would have gone "Yeah I thought that was all of it too", letting him back out of it easily.
Why would they think less of him for not buying the stuff they didn't know existed when he made the deal...?