Praise be to Allah.
What is meant by feeding (a poor person) is half a saa’ of the staple food of the country, such as dates, wheat, etc. This is equivalent to approximately one and a half kilograms of rice. See Question # 9985.
With regard to giving the price of the food instead of the food itself, the correct view is that giving the price does not fulfil the conditions of the kafaarah (expiation). This is the view of the majority of scholars. Ibn Qudaamah said in al-Mughni: “The kafaarah is not achieved by paying the price of the food or clothing, because Allaah mentioned giving food, so the kafaarah is not achieved by anything else. Allaah gives us the choice of three things’ if it were permissible to give the price, the choice would not have been limited to these three…” (al-Mughni by Ibn Qudaamah, 11/256)
See Fataawa Islamiyyah, 3/464; Kitaab al-Aymaan wa’l-Nudhoor, Muhammad ‘Abd al-Qaadir, 87.
And Allaah knows best.