What is the ruling on one who vowed to fast for an entire year but is unable to fulfill his vow?

A vow to fast for a full year must be fulfilled if the condition upon which the vow was suspended is met, or if the vow was absolute. This is based on Allah's saying: "And let them fulfill their vows," 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 the vow except in the case of permanent inability that is not expected to cease, in which case an expiation for breaking an oath becomes obligatory. However, if the inability is temporary, one must wait until it ceases. It is disliked to make vows, especially if they are contingent upon a condition, as the Prophet (peace be upon him) forbade it and explained that it "does not avert anything; rather, it merely extracts from the miser."