To be precise, he served as a delivery boy for Yuuko, to deliver said sake to a an old lady from another side of the city (the catch being crossing through a city with hordes of thirsty spirits, a mean spider-lady, and an evil eye spirit - all without harming a single being). It was after finishing the task when he was invited over for a brief drink, in which he was told of its' properties.

While it wasn't a purchase from Yuuko per se, given the existence of equivalent prices in xxxHolic, I'm alright with diminishing the ability if needed.