Praise be to Allah.
We must understand an important principle, which is that when Allaah specifically mentions giving food, then it must be food. Allaah says (interpretation of the meaning):
“And as for those who can fast with difficulty, (e.g. an old man), they have (a choice either to fast or) to feed a Miskeen (poor person) (for every day). But whoever does good of his own accord, it is better for him. And that you fast is better for you if only you know”
And He said concerning kafaarat yameen (the expiation for breaking a vow) (interpretation of the meaning):
“for its expiation (a deliberate oath) feed ten Masaakeen (poor persons), on a scale of the average of that with which you feed your own families, or clothe them or manumit a slave. But whosoever cannot afford (that), then he should fast for three days. That is the expiation for the oaths when you have sworn. And protect your oaths (i.e. do not swear much). Thus Allaah makes clear to you His Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) that you may be grateful”
In the case of zakaat al-fitr, the Prophet (peace and blessings of Allaah be upon him) enjoined giving one saa’ of food as zakaat al-fitr. So whatever is mentioned specifically as food or as feeding (the poor) in the texts cannot be given in the form of money.
Based on this, if an old man is required to give food instead of fasting, it is not acceptable for him to give money instead. Even if he paid ten times the value of the food, it would not be acceptable, because he has turned away from what is mentioned in the texts. Similarly with regard to zakaat al-fitr, if he paid ten times the value of the food, it would not be acceptable in place of a saa’ of wheat, because the equivalent value is not mentioned in the texts. And the Prophet (peace and blessings of Allaah be upon him) said: “Whoever does an action that is not part of this matter of ours will have it rejected.”
Based on this, we say to the brother who is not able to fast because of old age: Feed one poor person for each day, and you may give the food in one of two ways:
1 – Giving it to them in their houses, giving each one one-fourth of a saa’ of rice along with some condiment to accompany it.
2 – Making food and inviting a number of poor people whom you are obliged to feed. So if ten days have passed, you could make dinner and invite ten poor people to come and eat, and do that when the next ten days have passed and the next ten. When Anas ibn Maalik (may Allaah be pleased with him) grew old and became unable to fast, he would feed thirty poor persons on the last day of Ramadaan. End quote.