It is Not Permissible to Begin With Fasting Before Feeding (the Poor)

Question :

If I swore not to do something, then came a day when I did it, may I fast for three days, then complete this thing? Or must I stop doing it?

Answer:

If a person swore not to do something, then he did it, he must make the atonement for an oath. If he said: "By Allah, I will not speak to so-and-so," or: "I will not eat his food,"

then he spoke to him or ate his food, then he must make atonement for an oath, according to the Words of Allah, Most High:
"Allah will not punish you for what is unintentional in your oaths, but He will punish you for your deliberate oaths; for its expiation (a deliberate oath) feed ten Masäkin (poor persons), on a scale of the average of that with which you feed your own families, or clothe them or manumit a slave. But whosoever cannot afford (that), then he should fast for three days. That is the expiation for the oaths when you have sworn. And protect your oaths (i.e., do not swear much). Thus Allah makes clear to you His Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.) that you may be grateful.

In this Verse, Allah, Most Glorified explained the atonement for an oath. He, the Almighty, the All-Powerful has explained that fasting is only the right of a person who is unable to feed or clothe the poor or free a slave. Some of the scholars have disagreed over the required amount of food for each poor person. The most correct opinion is that it is half a Sa' of any kind of food a man feeds to his family, such as rice, dates or anything else. The amount of it in weight is approximately one and a half kilograms. If he fed ten poor, lunch or dinner, or clothed them in garments suitable for prayer, that would be sufficient. If he freed a believing slave - whether male or female - that would be sufficient. And if he was unable to do all of these things, he should fast for three consecutive days.

And Allah is the Granter of success.

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 6 Pages 398-399