The World Federation One Stop Fiqh

Ruling 2664

A person can only vow to perform something that is possible for him to perform. Therefore, if a person who, for example, is unable to walk to Karbala vows to do so, his vow is not valid. If at the time of making a vow one is able to perform it but later becomes unable to do so, his vow becomes void (bāṭil) and nothing is obligatory for him [concerning this matter]. The exception to this is if he vows to keep a fast, in which case if he cannot do so, the obligatory precaution is that he must give 750 grams of food to the poor for every day [that he had vowed to fast but was unable to], or he must give 1.5 kilograms of food to someone to fast on his behalf.