Question: What can I do if I have broken an oath of ibada? I made an oath to fast every second day and I was not able to continue. Do I have to start again doing every second day or is there an expiation I can do in this case? I have heard that if its an oath to do with a promise to do a certain ibada then you must fulfill it? Also, this is too many fasts for me to make up, so what should I do?
Answer: To begin, an oath to make a non-obligatory action (nafila) obligatory is called a Nadhr (نذر) and is recommended to do if it is a one time act. If the nadhr is to do something on a regular basis, then it is disliked (makruh) according to the Maliki school. In any case, once a person makes the nadhr oath, then they complete it or pay an expiation (kaffara) if they do not do it or break the promised habit.
If the nadhr was a regular fast and not fulfilled, then the person would have to pay an expiation (kaffara) even if the person went back to the fasting. Additionally, once the nadhr is broken and the expiation is paid, then the nadhr is no longer binding. The expiation (kaffara) is one of three methods:
1) Feeding ten poor people each a mudd (0.5 liter measure) of a staple grain (it is recommended for those outside of Medina to give 1.3 mudds to each miskeen), or giving them each two loaves of bread weighing two Baghdadi ritls for each loaf, or satiating each poor person (miskeen) with two separate meals for each person. The definition of a poor person (miskeen) according to the Maliki madhab is a person who does not have food for more than one day.
2) Clothing ten poor people. For each man a thobe is given. For each woman a thobe and and hijab. A child may also be included in the ten.
3) Freeing a slave as a person would do in dhihar.
If all of these three are not a possibility for the person, then a person can fast for three days consecutively.
And Allah knows best.
source : http://seekersguidance.org/ans-blog/2013/06/27/expiation-for-breaking-an-oath-to-fast-in-the-maliki-school/