When it came to discussing the price, Yang Yi knew that the deal was pretty much secure.
Pyatov went to the warehouse with the now-dead Eugene to fetch guns. It wasn't a big deal, but it was enough to show that Pyatov was undercutting his brother-in-law.
The saying goes, a dam is destroyed by ant holes. Many big issues start from something trivial. Right now, Pyatov stealing a few guns to satisfy his needs wasn't a big deal. Even taking dozens or a few hundred at a time didn't matter much because, for someone like Kanchelskis who handled large quantities of arms, a few hundred guns were nothing.
The problem was that Pyatov stealing these rifles to give to the gangs was definitely wrong and could cause trouble for Kanchelskis, yet Pyatov did it anyway. That meant he could backstab his brother-in-law in even bigger matters.