Qaḍāʾ is fasting later in place of a missed day. Fidyah is provision of a prescribed food allowance under particular conditions. Pregnancy/nursing-related temporary omission leads to make-up; lasting old-age inability leads to fidyah; a deceased person’s qualifying bequest leads to estate-funded feeding.
These are not interchangeable routes. A living person capable of fasting does not normally extinguish a make-up duty simply by donating food or money.