The Gemoroh is actually the first mishna in the third perek of Nedarim.
It states that four types of neder don’t even need a shailoh asked, but can be “broken” without any violation.
One of those is a “neder zairuzin” – a neder made to pressure someone to hurry up and deal.
The case given is that the seller makes a neder that he won’t sell for less than four, and the buyer makes a neder that he won’t pay more than two. It’s explained that since each really meant a price of three was acceptable, they can make the deal.