Amatul Rahmân Omer:
Allâh will not call you to account for (such of) your oaths as are vain, but He will call you to account for (breaking) the oaths which you take in earnest (for doing a thing or not doing). The expiation for breaking such an oath is the feeding of ten needy persons (the average food) as you feed your own families with, or provide them with clothes, or liberating of some one (from the yoke of slavery or debt etc.) But whoever does not find (the means to do any of these), then he shall fast for three days. That is how you expiate your oaths when you have sworn them solemnly (and then break them). But do guard your oaths. Thus does Allâh explain to you His commandments so that you may give (Him) thanks.
Surah Al Maidah (The Table Spread with Food) Ayat 89