Is it obligatory to atone by feeding ten needy people, or is fasting sufficient, considering there is 700 pounds available, that the expiation for an oath is 100 pounds, and that the age is 16 years old?

The inability that permits transitioning to fasting is that the one expiating does not possess more than what suffices for their own sustenance and the sustenance of those whose maintenance is obligatory upon them for one day and one night. If they possess more than that, then feeding is obligatory upon them, and fasting is not sufficient for them. They are not obligated to save from their expenses or Eid gifts in order to expiate by feeding. Therefore, expiate by feeding with what you possess, and for the remaining expiations, it is legislated for you to fast them.