Is it obligatory to fulfill a vow before the wish is granted or after it, and what are the consequences of not fulfilling it after it has been granted?

A vow contingent upon a condition is disfavored in Islamic law. However, if a person makes such a vow, and its condition is fulfilled, and the vow itself is an act of obedience to Allah, then it is obligatory to fulfill it, for the Prophet (peace be upon him) said: "Whoever vows to obey Allah, let him obey Him." If one is unable to fulfill it, then expiation for breaking an oath is required. It is permissible to fulfill a contingent vow before its condition is met, but it is not obligatory.