The mishna, N'darim 4:6, in my own, very loose translation:
Aaron asks Baruch, "Lend me your cow."
Baruch tells him, "It's not available."
Aaron says, "My field is forbidden to me like a sacrificial offering for plowing it with your cow."
If it's his practice to plow, himself, then he cannot plow his field [with Baruch's cow, since that was what he meant when he forbade it], but others can. But if it's not his practice to plow, then [he meant that even others can't use Baruch's cow, so] neither he nor anyone else can [plow it with Baruch's cow].
Most of the other cases until this point in N'darim are not so detailed with respect to the background of the case. I'd expect something more like:
Aaron says, "My field is forbidden to me like a sacrificial offering for plowing it with your cow."
If it's his practice to plow, himself, then he cannot plow his field [with Baruch's cow, since that was what he meant when he forbade it], but others can. But if it's not his practice to plow, then [he meant that even others can't use Baruch's cow, so] neither he nor anyone else can [plow it with Baruch's cow].
— i.e., missing the initial conversation about the attempted loan. For example, earlier in 4:6 is a case of someone who forbade himself his fellow's benefit (like here), but it doesn't say why. And the same is true in 4:5, 4:4, and numerous times in the preceding chapters. Why here does the mishna record a conversation of background information as to why Aaron forswore Baruch's benefit?
