Is it permissible for someone who vowed to cook for their sister to prepare two dishes of food for the worshippers in a specific mosque to change the vow by cooking it themselves, or by giving it to another mosque, or by substituting it with money for the poor or for building a mosque, given that the proposed substitute is of greater value?

A vow contingent on a condition must be fulfilled if the condition is met. If the vow was made for the people of a specific mosque, it is not permissible to spend it on others. The vow must be fulfilled in the manner in which it was made, and it is not permissible to substitute it unless it is better and more beneficial for the poor. Therefore, it is not permissible to spend the money on building a mosque or giving it to the people of another mosque. Rather, the food must be prepared and given to the people of the specified mosque, and it is permissible to give them the money if it is more beneficial.