What is the ruling when one swears not to do something, conditional on the will of another, and the other expresses a will to refrain?

Primary text

If a person swears, "By Allah, I will not drink today, if Zayd wills it," and Zayd says, 'I have willed that you not drink,' and the person then drinks, he violates his oath. If he drinks before Zayd's will is established, he is not in violation because the abstention from drinking was conditional upon Zayd's will, and since Zayd's will was not established, the abstention was not established. This differs from the previous case. If Zayd's will is hidden, it is treated as non-existent.