What is the ruling on breaking an oath when one says: "If I do not do such and such, then I will buy a sheep for our friends"?

If what occurred was a promise to sacrifice a sheep for not doing a certain thing within a month, and it was not done, then fulfilling the promise is desirable, not obligatory. However, if it was a conditional vow and the condition was met, then it must be fulfilled, for the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him; and whoever vows to disobey Allah, let him not disobey Him."