Well, if you have another in-universe character telling you this - from a quick Google, looks like 'Crafty Xu' is a merchant? - then you have several different possibilities, not all of them being mutually exclusive. Is Xu under the influence of Qian Ya, or another Yama King? Is she relating the Chinese tradition, akin to the Faerie tradition, in regards to the Yama Kings? Is she telling the characters what they want to know in order to get them to shut up and buy her outrageously overpriced stuff? Does she think she's telling the truth, but it's incorrect, i.e. the 'unreliable narrator' issue?
All that said, 'has to honor a deal' is tradition in a lot of different backgrounds; in Christian tradition, the Devil has to honor the bargains he's made. Metagame-wise, it's just as possible that such deals for the Yama Kings are akin to minor (non-ritual) pacts, which she's going to want to keep because she gets energy, i.e. karma, out of it if she does, and loses energy (force, spirit energy, or karma) if she doesn't.
And in regards to the fae and telling the truth, traditions vary on whether they could directly lie to you, but all agree that they had to tell you the truth if you asked them three times.