What is the ruling regarding a husband's refusal to fulfill his vow of fasting two consecutive months without a valid excuse?

A conditional vow is disliked (makruh), but it must be fulfilled if the condition is met, based on Allah's saying: "And let them fulfill their vows" (Quran 22:29), and the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him." It is not permissible to deviate from fasting unless there is permanent inability, in which case an expiation for an oath (kaffarat yameen) becomes obligatory.