Is a person upon whom an expiation for an oath (kaffarat yamin) became due, and who was unable to feed [the poor] but later acquired money and spent it, absolved by fasting the expiation, or is he obligated to feed [the poor]?

Expiation becomes due upon breaking an oath. The criterion for expiation of an oath is the condition of the oath-breaker at the time of breaking the oath. Thus, if someone was in financial hardship at the time of breaking the oath, what is obligatory upon him is fasting. And even if he becomes financially well-off after that, only fasting is required of him, and his expiation through fasting will be valid and sufficient.