On their deceased father, for a number of days in Ramadan that is unknown

Question:
May Allah reward you. The questioner says: My father has missed fasts from Ramadan from many years ago, and we do not know the number of days. We want to be dutiful to him. What is required of us, his children?

Answer:
I mean, their father is dead. Yes, they should strive to estimate the days he owed, and they should be cautious and make them up. That will benefit him, with Allah’s permission, and it discharges the obligation on his behalf. A woman asked the Prophet (peace and blessings be upon him) that her mother had vowed to do such and such, and asked if she could take her place and fulfill this vow for her. He said: “Pay it off.” He said: “Pay it off, for Allah has more right to be paid off. Do you think if your mother had a debt, would you not pay it off?”
She said: Yes. He said: “Pay it off, for Allah has more right to be paid off.” So if the mother or the father or one of the relatives or one of the Muslims dies and he has fasting owed, it is made up on his behalf. Fasting of a vow or fasting of Ramadan is made up on his behalf, and that will benefit him, if Allah wills. Yes.