Praise be to Allah.
The majority of Maaliki, Shaafa’i and Hanbali fuqaha’ are of the view that paying cash does not count as the fidyah for not fasting, and what must be given is actual food, because Allah 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)” [Al-Baqarah 2:184]
Ibn ‘Abbaas (may Allah be pleased with him) said: This refers to old men and old women who cannot fast, so they should feed one poor person for each day. Narrated by al-Bukhaari, 4505.
It says in Fataawa al-Lajnah al-Daa’imah (10/198): If the doctors have stated that this sickness means that you cannot fast, and there is no hope of recovery, then you must feed one poor person for each day, giving half a saa’ of the local staple food, whether it is dates or something else, for past and future months. If you give a poor person dinner or breakfast for the number of days that you did not fast, that will be sufficient. As for giving cash, that does not count. End quote.
So the elderly or sick person for whom there is no hope of recovery should feed one poor person for each day, giving half a saa’ of wheat, dates, rice or similar local staple. That is equivalent to approximately one and a half kilograms. See Fataawa Ramadan, p. 545.
He may give it all in one go at the end of the month, 45 kilograms of rice, for example, or if he makes some food and invites poor people to come and eat, that is good, because that is what Anas (may Allah be pleased with him) did.
If you gave the fidyah in the form of cash following the view of a scholar who issued a fatwa to that effect, then you do not have to give it again. But if you did that yourselves, then you have to give it again, in order to be on the safe side and in order to honour your father, may Allah have mercy on hi and forgive him.
And Allah knows best.