Is it permissible to vow to fast a month and donate a sum of money if Allah cures one of bulimia?

Undertaking a vow (nadhr) is disliked. However, if you made a vow and were cured of your illness, you must fast for a month and give in charity the amount you specified, because a conditional vow must be fulfilled if its condition is met. If the cure does not occur, then nothing is incumbent upon you.