Is zakat due on a sum of money collected by the deceased father, knowing that it was designated for the expenses of the death and the condolence, and is zakat calculated on the value of silver retroactively?

Zakat becomes obligatory on wealth once it reaches the nisab (minimum threshold). If the father failed to pay Zakat due to ignorance of its obligation, believing his wealth had not reached the nisab, then there is no sin upon him, Allah willing. However, the heirs are obligated to pay Zakat from his estate for all the years it was not paid.