What should be done about a vow to fast for an unspecified number of days, and is it permissible to feed the poor instead of fasting?

If you made a conditional vow (nadhr mu'allaq) and the condition upon which you suspended your vow was met, then it is obligatory to fulfill it, due to the saying of the Prophet, peace and blessings be upon him: "Whoever vows to obey Allah, let him obey Him." If you are unsure about the number of vows or the number of days vowed to be fasted, then you are obligated by the certain minimum. It is not permissible to switch to feeding (miskeen) instead of fasting except in cases of inability to fast, and your not being accustomed to fasting is not an excuse. You must fast the vow, even if intermittently, unless you intended consecutive fasting.