Is it necessary to re-fast the expiation for an oath (kaffarat al-yamin) after attempting to pay it but being unable to (due to inability to fast)?

If you are unable to expiate (a broken oath) by feeding or clothing after fasting, then your fast is valid and nothing is required of you. And if you become capable of feeding or clothing after having started fasting, you are not required to revert to them. This is the view held by Ibn Qudamah and others.