If you're trying to unload an extremely rare high value good without any kind of connections, you'd be happy to get 26%. I mean, yeah, he could try going to a different town and asking for a better deal, but they might just say the same thing and it would be a huge hassle with travel expenses, time and the dangers of the road. Also, he has this skill now so he can easily acquire more high value materials, so he doesn't really have an incentive to try to spend time and energy on getting the best possible deal for that one mandragora. He'd be much better of accepting 26% and going out and collecting a bunch more materials instead. Also, what's he even gonna do with all of that money? He just wants his sister to live a comfortable life, and I guess he needs to upgrade his adventuring gear, but that need will be satiated quickly at the rate he's going. It'd be a different matter if he was running a business and needed capital for reinvestment, but with his lifestyle, he can afford a little loss.
