Praise be to Allah.
It is not sufficient to fast when one is able to feed or clothe poor persons or free a slave, because Allaah stated that one should fast only if one is unable to feed or clothe poor persons or free a slave. He says (interpretation of the meaning):
“Allaah 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 Masaakeen (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”