Is it obligatory to offer expiation by feeding ten needy people or by fasting if one cannot afford it?

The expiation for an oath follows the order mentioned in the noble verse. If you fasted as expiation for your oath and completed it, then later doubted whether you had money to feed people or not, there is no blame upon you. This is because doubt about an act of worship after its completion is not considered, especially if you are prone to obsessive thoughts.