I predict all these people predicting a fight haven't read much litterature to have ever seen a different outcome in situations like this. I'm not saying it's not going to end in a fight, but seriously, if you're going to make a prediction, you need to be 100% sure between a few choices rather than 70% on a single one.
You don't even know wether the rapier is made from an excessively light material, or has some magic on it, or both. What if the sheath only opens for a specific bloodline ? I won't bother enumerate all the risks involved for the merchant if he were to try to retake the item. Also, the MC even talked about losing face as a merchant and blah blah, heavily hinting the guy respects the rules even though he's shrewd about interpreting words. What this means is, the rapier has been bought by MC, deal is closed, no re. Of course the author could have the same mindset as his readers and not give a fuck about making sense, in which case you can forget about this post.