Is it permissible to combine the intention of fasting a vow with the fasting of Ramadan, or with voluntary fasts such as those on the days of Shawwal, the White Days (Ayyam al-Bid), the Day of Arafah, and Ashura?

A Muslim must avoid sins out of fear of Allah. If your promise to abandon a sin was merely an intention and you did not explicitly utter a vow to fast, then, according to the majority of scholars, nothing is binding upon you. However, if it was an explicit commitment to fast one hundred days if you return to the sin, then it is an oath of obstinacy and anger (Nathr Lajaj wa Ghadhab), and you are obligated to do one of two things: fast one hundred days, or offer an expiation for an oath (Kaffarat Yameen), which is feeding ten poor people, or clothing them, or freeing a slave; if you are unable to do any of these, then fasting three days. If you choose to fast the one hundred days, it must be with the intention of fulfilling the vow (Nathr), and it is not permissible to combine the intention of fasting these days with the intention of fasting Ramadan or voluntary fasts.