127:12 If a person obligated himself to fast and he became very distressed during the fast, he can redeem his obligation using money according to his means; this money is given to the poor. (The reason this is effective is because the pain of the financial loss takes the place of the pain of fasting, which is why it’s commensurate with one’s personal financial situation – Mishnah Brurah 568:24.) If a fast is undertaken because a vow, it cannot be redeemed. Similarly, if a fast is decreed by the community, it cannot be redeemed unless a condition permitting this was included by the community.
127:13 If a person vowed to fast on Monday, Thursday and Monday, he is allowed to change this and to fast on Thursday, Monday and Thursday, but not on any other days. This is because because the presumption is that his intention was to fast on the days of judgment.